Honestly, I doubt you will need much heating overnight there in July. However, we've used our furnace now and then and it seems to be pretty frugal on propane usage. My trailer has two-20# tanks, does yours have only one, at 15#? We use propane for the refrigerator, hot water heater, some cooking inside (but most is done outside on a propane grill), and, occasionally the furnace to take the chill off in the morning. This has carried us through two weeks without going through more than one tank.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew