Not familiar with your coach, but most older Class Cs had the coach battery under the hood with the chassis battery. If not, it's somewhere in the coach. Yes, you have 2 batteries or should.
It does sound like there is a chassis battery disconnect. I doubt it was standard, so a previous owner could have put one in.
I would first remove the battery cables to make sure they are clean and making a good connection. Then follow the ground cable to the frame, doing the same there. Now you have eliminated the major causes of you problem.
Other than that, you just have to trace the batt cables to see if you come across some sort of disconnect switch. Good Luck!
John & Clare - Buck & Zeus - Jack & Husky
2008 Fleetwood Bounder 38P
Towd: 2 HD Streetglides