I think your truck will work, others do it and are actually still alive and post on this forum. however you are right on the fence.
Isn't an 855s about 4k# plus, rolling down the road ?
240# for all your stuff and water ? All your fishing EQ, plus the food, 6 gals of water in the water heater, full propane , wife and dog ???
Are your bringing a real person and dog or is it just a picture of them ?
Some people actually add beer to that number and that can really blow the truck apart.
Dry weights do not incl. the campers option list.
I would be more concerned about axle weight ratings opposed to dry weight or payloads because most of the camper weight is carried on the rear axle.
The rear axle is usually the limiting factor.
due to tire and/or wheel ratings.
Maybe some 855s owner with a real CAT scale weight would comment . But I would think a loaded slide camper of any brand is going to be 4k.
If you plan to go on the beach, I don't think the 19.5" tire idea will work. Can't air them down without damage. Most guys that do go with 19.5" for highway travel have a second set of 16/17/18" just for the sand.
Bob