Best Method for Propane Level guesstimates?

I'll have to see how much propane the furnace uses in the early season. If all goes well we will be going to the beach every opposite weekend, starting on the 15th.  The cottage is near the beach inside a bay, the temps are as much as 10F higher than 50 miles inland this time of year. If the wind blows from the south it is warmer, if it shifts from the north it can still get chilly at night. If we use a lot, then double tank mod gets bumped up on the list to get done before fall.

I have a tank gauge because I don't feel like unstrapping/unbolting the tank.  When we camp, we don't use the heat and electric for the water heater, so the only gas we use is for cooking.  I've camped 7 or 8 weekends and still have over half a tank left.  

I have two tanks, when tank one is empty I hook up number two and keep my eyes open to refill number one

Thanks for letting me know those tanks with built in are useful enough, I may go this route. Once it reads zero I can give it a swish and evaluate. I could get two tanks, for swap out, and use the remains on my grill or something.

Well warm water it is then! That should be easy enough at home.  Definitely better than weighing the tank.