Don't think 18" tires come with high load ratings, that's is why those who used SRW for heavy campers had to upgrade to 19.5" wheels.
Coming to legal part, is your truck register as private vehicle or as commercial one?
In most of the states it is commercial registration and law require driving via DOT scales. Nobody does that with camper on and it is not reinforced, but again, we are talking legal. Than depends on the state, some DOT scales say vehicles above 11k weight, some 20k and so on.
Some states like California do have weight stickers, who in big size are displayed on side of truck. When DOT doesn't mind to register 1-ton truck for 30-40,000 lb GCWR, the tickets for underpaying the registration are pretty hefty.
Suppose only some Canada provinces actually weight pickups, but unheard in USA.
In the past I was using F450 flatbed for equipment hauling.
Only time DOT officer put me on portable scales was in Montana. He evidently was bored.