You have some things working in your rig that works on draining the battery. The water pump will do it as it will draw a good amount of amperage from your battery. Lights, the refrigerator control board, the LP gas monitor and anything that uses battery power will do it. Maybe you have a compartment light on that helping the process also.
First of all, the battery that comes in these units aren't true deep cycle batteries so they won't last as long as a real deep cycle battery will. You can add another battery to keep you going longer. The new one should be as close in capacity as the OEM battery. The connection is simple, a parallel connection is all that is needed. You will need two more cables to do this. Most auto parts store can make you up the interconnect cables that you will need.
You won't have a battery vent cover with the added battery but you can turn the OEM cover sideways and that will help until you can find another cover.
Serial and Parallel Battery Configurations and Information