Somewhere either here or on line I saw a block diagram of the typical RV 12VDC system. While a block diagram doesn't show individual components they are usually detailed enough to determine test points. If you are DIYer, get the block diagram and an inexpensive Digital Mulitmeter(DMM) or an analog meter, and learn how the system works. I am far away from my rig, over 5 hours by air so I can't send what I have at home. Checking stuff out is pretty easy if you know were to look. There are a lot of places to visit and get a load check on the batteries for free. You should do that too. I use the shop that services my rig or AutoZone, both are in my opinion trustworthy, if batteries are dead, I go to Costco for new ones.
Good Luck and let us know how you faired.