The OP's "18 gallon" propane tank probably only holds around 80% of that when filled - about 15 actual gallons of propane. At least that's the case with our 24 foot Class C's 18 gallon (water capacity) propane tank.
Our 15 gallons of actual onboard propane requires refilling fairly often, as we use our propane furnace to keep warm nights when drycamping. We don't care to sleep and get up in the middle of the night in the cold, so we keep the coach interior temperature in the 60-65 degree range.
Our 15 gallons of onboard propane is a long way from being enough to last for months.