We have a similar size 5th wheel and swaped out for a 2001 V10 F250 last year. Haven't put a lot of towing miles on it yet but plenty of power. Getting around 9mpg so far.
We haven't had the oppurtunity to do any significant hills yet. Just expect to wind it up and run in a lower gear. You should have no problem keeping up with the semi's in the right lane (should be able to beat them but if you pull behind to let cars pass, regaining speed will be hard).
Check your payload numbers to be sure but I don't expect it to be a whiteknuckle drive as someone suggested.