Wise to follow the majority here. Personally, I wouldn't go over 7k wet with an F150. That's right about where I was for the last few years. It did a phenomenal job, but if I went any heavier it would have been an "OK" job.
Mileage on very long trips was between 9 and 10. At best, you will save a couple of hundred a year (if that) by switching. And, you will spend much more money to "save". You also give up all of the benefits of a motorhome. Didn't seem like a good idea to us, so we just switched in the other direction. :) But there are 31 flavors for a reason.