A bit more reliable than the level gauges for the propane tanks is the auto-switchover. It should visually indicate when it's switched over to the 2nd tank, which means the 1st one (the one the selector is pointing to) is empty. You can then change the pointer over to the 2nd tank, remove the 1st tank and go refill it while the furnace, water heater and fridge can use propane from the tank that's still on the trailer. If you have the fancy tank cover, it may be a good idea to get the fancy remote indicator for the switchover valve so it doesn't rely on you checking inside the tank cover every day or two to look at the regulator.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill