I know this NOT a building, but, from a structural engineering perspective, the UBC (Uniform Building Code) limits deflection in a beam/roof/floor to L/240.
L = span in inches
So if your RV roof spans 8 feet, (side to side) L = 8 x 12 = 96 inches
96/240 = .4 inches or about a half inch, which, FEELS about right.
1 1/2" seems a little high.
Industry standards ahhhhh hell, who needs 'em?
Second thought: If you could get the floor directly below to deflect the same amount, . . . . . problem solved! No deflection, that's called camber. Then drill 1" holes in the roof and your drainage problem is solved too! And you get a free rainwater catch system.
Chum lee
