Fords come in three different cab sizes and two different bed lengths resulting in six different wheelbases. Although I was very happy with my F250 extended cab short bed, it did limit which truck campers I could purchase. Our Arctic Fox 811 is very well built but we want additional features that a F250 short bed really could not handle well. I have since upgraded the truck to a Ram 5500 with 9' deck that can handle the largest TC or FW RV currently produced - When we bought the F250, we never considered these RV's as possibilities (our wants have changed).
I would not suggest the F450 pickup over the F350 for TC use since it actually has less payload available. If you need additional capacity look at the Class 4 or 5 chassis cabs as the next step. I don't suggest you run out and buy a Class 5 diesel, but I do suggest figuring out which RV suits you and scale the truck up enough to allow an upgrade in the future without having replace it. Having extra capacity to make changes or select a different RV in the future is much easier than having to replace TV and RV at the same time.