Keep RV Battery Charged
You first must calculate how much power you used, so you know how much you need to replace. Your heater will be rated in watts or amps. Multiply that number by the estimated number of hours the heater ran, and you will have your baseline number. Once you have that you can calculate what size panel you need. You can use our solar calculator once you have that information.
Before you do, keep in mind that solar only charges when the sun is shining on the panel, which is roughly 6-7 hours per day average. To charge a discharged RV sized battery charger with a solar panel in 6 hours would take a couple 100 watt+ panels