Suburban SW6 lights but drops out after 30+ seconds

Ummmmm... well food for thought isn't it ! I agree but my little Toy Hauler has always had about 12.5 vdc battery reading on my solar power regulator and when on shore power ? Same reading on my digital meter when checking wiring, etc.. Might be because of my old batteries perhaps ? Hasn't been an issue and when on solar panels I have power all evening (never use propane heater tho) and recharge during the day. Also have a generator if needed and always starts so .... I'm not sure ?