You may be worrying a bit much....if you figure that you will be getting 7 mpg, which seems to be the magic number with RVs of all sizes, you will be pretty close on your calculations...
Measuring the tank is still the best option. You may find that some of the fuel tanks are difficult to actually FILL...as they click off the pump hand-set much earlier than actually being full. (Something about the way the fill hose/tube goes down into the tank on some models.)
I don't think I would want to run the RV down to empty to test the gas tank. You are inviting problems with dirty fuel and such getting into the filters. I think it's a pretty good practice to try and keep the tank at no less than 1/2 tank, as this will also allow your genset to operate. It would pretty much suck to break down on the side of the road with less than enough fuel to run the generator....and you'll get almost 200 miles on that half tank, so you may want to pull over and stretch your legs anyway.
Have fun this summer:)