fuel mileage on 1995 Thor class A

When you do a tune up, take the time to do a compression check. That may give you some indication as to the real condition of the engine. I assume this is a new to you purchase. If so, many motorhomes sit for a long time before they are finally put up for sale. Could be the cylinders rusted and the rings are not seating very well. That may improve over time, once things get loosened up again.

RE-read your post couple times.

You sure your the math right? Are you going by some device put in by manufacture or by hand? Is odometer right?

If math is right, something is wrong to get that low of MPG. IMO

I could see dropping a mile or two MPG if badly tuned but 3.5 something is wrong.

Slow down!
Give the rig a good tune up!
Weigh your rig and see if it is overloaded. Put the rig on a diet!
You should be getting nearly twice the mileage stated.

1996 Southwind, 454 engine. 7 mpg in the western Pennsylvania mountains.
Take another trip and check the mileage. 3.5 sounds like something is not working properly.
Is it shifting into high gear? How fast were you traveling?

98 Damon Intruder 460 engine....8 mpg

We were pushing it bit not to many car passing me also pulling a car we have made other trips (slowed down ) got a little bit better mileage but still wasn't great (4-5 mpg).

We have a 1998 36' Southwind 460 Ford that we tow a Sunfire and we get 8 MPG
3.5 MPG sounds like something not calculating right

How fast were you running? The 460 was not know for great mileage between 7-9 at around 55 is very good! The faster you run the faster the gas goes!

A quick check on google indicates the distance is 204 miles, round trip 408 miles.
Posssible one tank fill.
Its not possible to get a good indicater of fuel mileage from such a small distance or one tankfull.
Sometimes when you fill you do not get a good fill and may me several gallons short of full and the next time you may fill more. depends on how level you are when filling your tank and venting of your tank itself.
I would venture to say you mileage is better than you are assuming.

I have a '93 Frontier Flyer 32ft.. I take it to Talladega.. and I get just at 8 to 10 MPG.. I put new plugs, wires, injector cleaner...general tune up.. it all helped...