I don't know about that Winnie, but some of the MHs have issues with the fuel getting down into the tank, and will shut off the trigger well before the tank is full. While it may not always be a good idea to play with the trigger to get as much into the tank as possible, you may want to fiddle with it at least once, to see what "FULL" actually is on this rig.
Once you know, I would then figure 7 mpg, and calculate the use based on mileage...say fill up when you have gone about 300 miles (42 gallons), which should give you plenty of "spare fuel", and you can try and figure from there.
I have a faulty gauge in my car, and I've gotten to where I don't really trust it, so base my use on miles. Just make sure to give yourself enough extra so you don't end up on the side of the road (like I did once).
Also...double check on that fuel tank capacity....it may be wrong?
good luck and Happy Camping