I think you will find that your propane tank will last a lot longer than you think. Get a separate propane grill for cooking outside the camper - it will help you camper's tank last longer, you won't heat up the camper every time you cook, and it will help keep the inside of the camper cleaner. Something like a Coleman Road Trip or Weber Q grill.
Our setup is a little different - we have two 30lb tanks mounted on the tongue of our trailer. When we camped more often (a few years ago, we were going out almost every weekend during the season) we never ran out of even one tank. So a 30lb tank lasted a family of four for 10+ weekend trips - this is using a separate outside grill for a lot of cooking, but running a water heater and everyone taking showers at night.
Look into getting something like that 1 gallon tank as a backup. You should be able to find a spot to store it on the trailer if you don't want it in your vehicle.
