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
I did a quick Google Maps search....from Myersdale to Savannah, and it offered two options (in addition to an airline...LOL!!) You can basically go west and take I-77 down or go East and take I-81 south.....or take I-81 down towards Richmond, VA, and then cut over to I-95 for the remainder of the trip south.
A lot will depend on what you want to see. I you are trying to get south as quickly as possible, it looks like western route down I-77 would be the quickest, and may also be the most scenic. I think that is what I would recommend, but I've never taken it, so can't really say for sure. Others will probably have better information.
That sounds like a great truck for the trip...the Hemi should have the power you need. Have a great trip!!