You didn't say why you prefer a 250 over a 350. I initially went shopping for a 250 Supercab long bed but found out that a 350 crewcut dually did not cost that much more but gave back so much more for the $$$. This would be good information for you to have as you make your final selection on that long 5er. IMHO I think keeping the weight down will be a challenge for you and will narrow your available units.
Also realize that a 40+ foot 5er will not fit in some campgrounds so you may want to review campgrounds where you plan to visit.