LP tank size

8.5? Who knows, probably some mfg ID. More than likely the dealer added a couple of gallons of gas to check systems. Take them to a true propane dealer, have them purged and filled. Commonly called 5 gallon tanks is the norm on trailers. Rarely 7 gallon tanks will be installed.
Heck, with our old trailers two tanks would last us all summer.

No actually a 20 pound tank holds 20 pounds of propane.
Gallon of liquid propane weighs 4.2 pounds.

No idea on the 8.5 but post a pic including all the markings. Could be the empty weight but that seems light.

Yes this was our first trip, We only had a fridge one oven use, a C-Pap machine and hot water heater. What does the 8.5 number on the tank mean?

Light use and only 4 days with two 20# tanks doesn't sound right.
My bet is that they weren't full. Is this the first time you have used these tanks? Possibly they were not "purged" and filled properly.

A 20# tank holds about 4 gallons of LP. Two of these should be more than enough for 4 days unless you were running something requiring a lot of propane like an LPG fired generator. Are you sure they were both full and both are now empty? Most newer trailers have an automatic changeover for the tanks to keep the propane flowing. When one is empty it changes to the full one (if both valves are open).