nguyenbio -
If I understand your problem/question, then the amount the truck squats will matter as to whether your camper is level when towing, but it will not affect the truck rail to fiver clearance. That is set by the hitch height in the bed.
I had the same problem with high truck rails. Just have to raise the hitch and pull nose high. Yes, if it squats more (250) then the camper will be more level, if it squats less (350), then the camper will pull higher. But again, the clearance won't be affected by the squat.
My truck rails are about 58.75" high. I ended up with a 5.5" clearance and pulling about 3" nose high. Wish I had more clearance, but haven't had a problem.
Anyway, use the opportunity to get the 1 ton for the reason you stated - to be ready for the future.