I think I'd pick the 350 just for the tires and consider the MPG the icing on the cake, assuming your camper is well within the 350's ratings.
19.5 tires are stiffer and heavier. That's great at 70 mph going down the highway but commercial traction tires don't have the grip that a light truck tire will have in bad weather, on wet, snowy roads. Part of the fun in having a truck camper is being able to go out into those kinds of places. You will be more comfortable in a truck with 17 or 18 inch tires under those circumstances.
If I had to have 19.5 tires to enjoy my camper, I'd find a M&S rated set and run them. Since I don't, I can enjoy my 1 ton with 285 width tires, softer rubber, more siping, and a grippier tread pattern when I'm off the beaten path.
I wouldn't deny that 19.5 tires are tougher in every way, but the tradeoff is less traction for that toughness. It'd be great if we could have both at the same time, but narrower and stiffer tires don't offer the traction that squishier ones do; it's a balance.
