There are a number of reasons that RV batteries go dead before you expect them to.  The most common reason is that people don't realize how much power it takes to run the average RV.  In some cases, people complain that they "have everything turned off but the batteries still go dead in just a few days".  Most decent batteries will not lose their charge for several months IF there is no load on them.  So, where did the power go?

In some cases, people don't think of the things that are drawing battery power even when you THINK they are turned off.  The normal kinds of things that people forget are the memory in the AM/FM radio, the propane detector, the propane solenoid on the propane supply (typically only in motorhomes), the TV antenna amplifier, the moisture free feature on some refrigerators pulls a LOT of power, compartment lamps that were left on, the control circuitry for the hot water heater, inverter stand-by current, the stand-by current for automatic DC power switching devices, etc.

There are also some pretty subtle places the power can be going.  I have heard of dash instruments that were inadvertently wired to the house batteries.  One of the motorhome manufacturers had a service bulletin on the fact that they had (mistakenly) wired the headlamp relay to the battery side of the ignition switch.  If you happened to shut down with the headlight switch in the high beam position, the relay COIL was continuously activated and would eventually run the battery dead.  Bet that one was fun to find!

Investing in a good digital ammeter is certainly worthwhile as it will allow you to see exactly how much current is flowing in each circuit as you test them.  Most of the digital meters out there today have both current (amps) and voltage settings.  You want to be using the CURRENT setting for these tests.  You can get an inexpensive digital meter from Radio Shack or many other suppliers if you don't already have one.  I recently picked one up at a computer store for $6.00!

Unplug the rig from shore power (or turn off the generator) and then turn off  EVERYTHING  in the rig.  Then start pulling DC fuses one at a time.  Start testing by touching the two wires from the lamp to the two contacts where the fuse was plugged in.  If the bulb doesn't glow, the current is low enough to use the AMMETER portion of your meter.  Connect the ammeter across the socket you just tested with the lamp.  (see picture)  Start with the meter set to a high range like one (1.0)  amp (just in case the circuit is using a fair amount of power) and read the current flowing in that circuit.  If you get no reading, start lowering the range setting as far as you can and read the current flowing in the circuit.  Most will read essentially zero.  When you find one that is drawing some current, check out what is on that circuit and then start isolating things until you find exactly what is drawing the current.

If you get through all the DC fuses and you STILL haven't found where the current is going, you can pull one of the battery cables and use your lamp and then meter there to determine how much current is flowing out of the battery.  Make SURE you start with your meter set to its highest current setting to do this test.  You can destroy the meter (or at least an internal fuse) if you overload the meter.  If you do this, MAKE SURE you have the rig disconnected from the 110 volt supply as you will see the CHARGING current from your converter that is flowing INTO the battery if you are plugged in and that can be very confusing.  If you find current flow there and you didn't find any at any of the fuses, you can almost BET there is a circuit that comes off the battery between the battery and the fuse panel.  Start your search and you CAN find it.  If your problem involves the starting battery of a motorhome, remember that typically the fuses that protect those circuits are not located with the house fuses inside the motorhome.  Typically they will be found either under the dash or perhaps under the hood... or both.

Remember it doesn't take much current over a period of time to take a battery all the way down and doing that to any battery will certainly shorten its life.  Even one amp being drawn continuously from a pair of 6 volt golf cart batteries will kill them in about 8 or 9 days.  They won't take this very often and you will be buying new batteries.  For this reason, it is a GOOD idea to have a switch that ABSOLUTELY disconnects or turns off the batteries when you are storing or not using the RV.

Battery Main Power Switch
There are a number of heavy duty switches available to do this job and just as many opinions as to whether it should be installed in the hot lead of the battery or the ground lead.  I won't get into that debate... there are good points on both sides.  In the typical travel trailer, it is unusual to pull more than 30 amps from the battery when operating and there are heavy duty toggle switches available at most auto parts stores for under $10.00 that will easily handle 30 amps with very little voltage drop.  In my travel trailer, I had a pair of 6 volt golf cart batteries on the tongue of the rig and a single 12 volt deep cycle battery under one of the seats.  I had a 30 amp toggle on each bank that was located under the kitchen sink, right near the converter.  In that way, I could connect one or both to the trailer if I wanted to or run the trailer on just the converter if we were parked with power.  I also had a 30 amp ammeter mounted near the switches so I could monitor the current from the batteries and also monitor the charging current from the converter going into them.  This setup worked well for many years.

In a motorhome, the switching requirements can be a bit more difficult.  In many cases, the motorhome will have an inverter that can draw a LOT of current.  In some cases, the total current draw that is possible on the batteries will be in the hundreds of amps.  In that case, and assuming the rig doesn't already have a master disconnect, you can add a switch (or switches) to do the job.  There are various types available for this job but one of the best is the type used in boats.  Many of these are water resistant which makes them well suited for the RV environment.  In addition, some of them can be found that are sealed so there is no danger of switching them near flammable atmospheres.  *SEE CAUTION BELOW!*  Make sure you get the right switch for your application.  In the case of current handling, when in doubt, bigger is always better.  One source for this type of device is http://www.westmarine.com/ In applications that have multiple battery banks, like most motorhomes, make sure you have a disconnect for ALL the battery banks.  The left picture shows a disconnect that would be used to select or disconnect two different battery banks.  The one on the right is used to disconnect the starting batteries of a diesel coach.

It takes time to find the parasitic current draws but for those of us that enjoy boondocking, this can pay large benefits.