No A/c power in my 2619...

I have a TM2619, 1999 model, that I picked up for a couple hundred bucks. I have no a/c outlets that work on battery alone. They all work fine when plugged in to external power. This has never been an issue before, I've never used them. Now, however, I want to take my father in law clamming, and he uses a CPAP forced air machine when he sleeps. Is there a switch I have to throw? or is there a way to check the inverter? Pretty new to this, and don't know where to go with it. Suggestions? I tow with a 2006 Honda Ridgeline.