Propane lasts how long?

Siting here in N. Texas where the WX isn't cooperating with my idea of spring, the furnace on our new 5th Wheel has only been running for a few days. We were gone all day and the camper was cool when we got back. I checked the stove, and sure enough, no flame. I changed to the other tank and all is OK. The first tank showed mositure up just under half way. Shouldn't it run itself out?

Advice please. I've been using diesel burning hydro Hot for years and this is all new to me.

Thx