There is another possibility; you could have a faulty gauge. Or you could be reading it wrong.
My Interstate holds 14 gallons or so in an 18-gallon tank. That's near enough to 80% as makes no difference. When mine gets down to about 1/3 full on the gauge, that's actually HALF full; it will take between 6 and 7 gallons to fill it back up to 14 gallons.
The only time your gauge will ever show full is if you fill the propane tank in cold weather, and then not use any propane until summertime. Expansion of the propane liquid will bring the gauge far enough above 2/3 full for the "full" light to finally come on.
If you run your generatoró and nothing else that uses propaneó you should get about 24-28 hours of run time before the generator starves. If you're running anything else WITH the generator, you'll get less run time. If you're running anything else INSTEAD of the generator, you'll be able to go days and days on 14 gallons; the water heater, stove, and furnace together don't use as much propane per hour as does the generator by itself.
Engineering: Finding complex solutions to simple problems you didn't even know you had.