OP: Your existing tires don't have "4-plies," maybe 2, nor will an"10-ply" tire have ten. That's an archaic construction and rating standard -- somewhat convenient, though. It's actually a "ply rating" with no direct influence on tire construction but a higher "ply rating" does indicate a more capable tire.
As Boomerweps notes the tires currently on the truck will handle the GVWR (fully loaded weight rating) of the truck just fine. The Cargo Carrying Capacity placard in the driver's door jam shows the maximum weight and required tire pressure. That includes the slide-in camper. More tire or tire pressure won't increase that. Going from a 4-ply rating to 10-ply rating will increase tire capacity but not axle or CCC capacity. Putting more air in the tire beyond the placard tire pressure will also make the truck ride like a truck.
-- Chuck