SumoSprings are only warrantied for 24 months, better to go with SuperSprings to get more support at the rear axle.
With 7500 lbs. of support at the rear tires and a truck that weighs 3340 at the rear you have a load capacity of 4160 lbs. and the Lance 855S has a dry weight of 2620 lbs. which leaves room for up to 1540 lbs. of water, food, and gear in the camper.
I don't see that you will have a problem or will need to go to 19.5 rims and tires unless you plan to tow a trailer that will have a 500 lb. tongue weight on the rear tires in addition to the camper's weight.
My Lance 845 has a dry weight that is a little over 2700 lbs. and when fully loaded for a trip the most it has weighed is 3400 lbs. and that included a half full fresh water tank.
I use a double leaf set of SuperSprings and tires rated for 3750@80 PSI and works exceptionally well.
I added a Big Wig anti-sway bar and it improved the ride when the camper is off the truck, no difference in ride or handling with the camper on the truck, though I have a GM truck which has a considerably stiffer frame than the Ford F-250/350/450 pickups.