We do weekend “dry camping” while participating in dog herding trials. My 27 ft class C MH is equipped with only one “house” battery. Although it is a deep cycle Interstate group 27 Marine/RV battery, it won’t last one day and over night, much less than that, if we have to run the furnace. The only appliance that we run is the LP refer., water pump and limited use of the house lights. I have had it tested and they tell me that there is nothing wrong with the battery. It was installed new on Nov 16th, 2005 and seems to take a full charge while running the gen. or traveling.
Two years for a battery sitting on a dumb charger without any special attention is pretty good. I’d say that it’s on its last legs. The “test” that the car parts place or even the RV shop does with the load tester doesn’t tell you anything about its capacity. All the test does is show its relative cranking capacity.
Yours is a fairly typical situation. You probably have a combination of the one battery that is past its prime, the OEM dumb charger and more loads than you realize. The house lights represent much more of a load than you realize, as each bulb draws at least an amp and many RV fixtures contain two bulbs.
First thing to do is to put an ammeter in line with the battery and determine your actual loads. The car parts places sell “inductive” ammeters that simply sit on the line and indicate amperage according to the magnetic field around the line.
Set up like you use it in the field, the same lights on, etc. and look at the current. Divide that current into the battery’s AH capacity and that gives you the hours available to 100% discharge. Not many, I imagine.
Next, install two or more batteries. That’s the cheapest solution. Sam’s sells a Group 29 battery that works well and current costs $69 bux. I bought 4 for my rig a couple weeks ago (04/2007).
The OEM charger/converter does a poor job of charging and it abuses the battery in the process. You’ll need a smart charger. How you go at this point depends on how much you use the rig. If the use is only occasional then you’ll want to look at one of the Vector/Black & Decker smart chargers available at Wallyworld, etc. The largest one outputs 40 amps.
If you’re using the rig regularly and are willing to spend a little more money, go with a Progressive Dynamics Intellicharge with Charge Wizard. Here  you can buy factory refurbs at excellent prices.
I recommend the PD9260CR 60 amp model with the charge wizard built in. Not only does this charger/converter charge the battery very rapidly but it also babies it with an intelligent multi-stage charge.
The reason I recommend the 60 amp model is that it draws less than 1000 watts which gives you several options in the case that you need to recharge during a trip. You can get a small generator such as the Yahama, Kipor or honda inverter gennys or if on a budget, this one.  I have two of these. I bought my last one last Christmas when Northern ran a special at $99. It is just right to run the 60 amp PD.
Another option for charging the batteries, the one I use in my MH, is to install a 1200 to 1500 inverter in your truck. Under $100. Plug the PD charger into this inverter with the engine running and it’ll charge the batteries as fast as possible and still baby them.
If you run a lightweight cord from the inverter to the charger then you can charge the batteries while you’re traveling. You could mount the cords permanently and install a plug/socket at the hitch so it would unplug just like your trailer lights.
The inverter will be handy for other things too – running a microwave at a picnic, operating power tools, running a TV, etc.
To know the state of charge of your batteries, that is, to know how much energy is left, you need a state-of-charge meter such as the Xantrex Link-10  Here is a photo. Click on the photo for a larger image.
This is the best couple hundred dollars you’ll spend to take care of your batteries. It’ll show you at a glance the discharge rate, consumed amp-hours, remaining energy and projected life in hours at the current average rate.
I’ve presented these items in a progression, from most vital to optional. After years of serious dry camping, I’d probably put the Link-10 right up there at the top. It is sooo nice to KNOW how much energy you have remaining instead of just guessing.