Newbie - electrical question, 12V not working

Just took delivery of a new TT 2 days ago, our drive back home from the dealer is 3 hours - and we parked in a RV storage lot. When I unhooked from TV I went to push the battery disconnect (at control panel by door) to disconnect before storage. However, first I tried turning on 12V appliances / switches and nothing lit up. It was a bit rainy, so I pushed the disconnect in and went home for the night. next day I went back to the TT pulled disconnect out and everything 12 V worked. However I tried again 15 minutes later and nothing.

Since it didn't work, then worked, then didn't work, I don't think it is a fuse or breaker. I am suspecting either a borderline battery charge or an intermittent problem (perhaps with the disconnect switch.) I put a meter on the battery terminals and read 13V (no load). Have not had time to investigate further. Sales dealership has been helpful on the phone, but I am not anxious to tow it into a repair place if I can fix easily myself - even if under warranty.

Any thoughts on what to try. pull off control panel and bypass disconnect switch? take battery to auto dealer and have it tested ? put volt meter to battery wires going into fuse panel (under refridgerator) and see if it reads 121 V when disconnect is pulled out?

Any advice appreciated. thank you.