Not only do you decrease effecive wheel torque by the %5 size difference, larger tires weigh more. 10 pounds or additional rotational weight per wheel has a far more effect than 40 pounds in the cab or bed. As noted by PMcColley, this affects acceleration, braking, handling and fuel consumption all negatively.
Don't do it. People who put bigger tires and aftermarket exhausts do not tow RVs. The engine load is far too high. What is acceptable as a compromise lightly loaded becomes obnoxious when towing a high walled RV.
