Multiple tanks are the failsafe (well, almost) approach. I start out every season with both 10 pound tanks full, and run off the tank that is easiest to remove. When that runs out, I switch to the other tank and fill the empty one as soon as practical.In my research for propane usage, a friend has the worst set up I think: 1 11lb horizontal tank. He (with a similar boat to mine with a deck box), opted for the one 11tank as it fit nicely in the deck box.
So he knows the BTUs in and roughly estimates the BTU's, burned. But in practice, it would be a job keeping track of burner hours.
He has settled on a number of days of use, then takes the tank off the boat to get it filled.
Do you feel lucky, go another few days, maybe roast that turkey,...No doubt you pull it off much more often and probably would have been better off with 2 5's.
Clearly, more than one tank is the best way to go, even if they are small like 5 pounders I installed.
It takes all of about 30 seconds to switch between tanks, as you just rotate the tank selector knob on the regulator assembly and open the new tank valve.
I don't think we've ever used more than two whole tanks during the 3-4 months we live aboard in the summer.
Pressure gauge on each tank is handy as a leak detector, but that's all it is.
If you have a reasonably accurate hanging scale or bathroom scale, it's easy to see how much propane is in the tank, since all tanks have the tare (empty) weight stamped on them. Just picking the tank up will give you a rough idea of how much you have left, if you have a reasonable sense of weight.
Frankly, I rarely check, since I always have a spare tank ready to go.
It's not a bad idea to carry spare POL o-rings, as these compress over time and can cause leaks in the long run. Make sure you buy o-rings of the right material, not just some o-ring that happens to be the right size. You can get those at most good hardware stores.