Bought my used camper last year and a solar panel was already installed when I got it. My last camping trip I ran the heater overnight for two nights with no problems and was camped under trees so do...
May want to disconnect the batteries entirely for about 6-12 hours (to let any charge voltage dissipate) so you can get a more accurate (SOC) voltage reading, or just use a hydrometer. With the solar controller (or any load) connected to the battery, any voltage reading is going to be suspect in terms of determining the battery's SOC.