The additional weight per Sq Ft with thin sheeting would not be of concern. If I decide to add a solar system, then I'll be looking to upgrade the axle. The current weight rating on the axle doesn't leave much room for more. I'll be moving to a Class A diesel in roughly 5 - 10 years for possible full time living, so I'm limiting add-ons to this T/T.
Adding higher rated axles does not increase the weight caring capacity. The frame rating will not change and cannot be changed by changing the axles. You might want to look into FlexArmor RV roof.