mine was on grass (which turned to dirt when the grass died under it) in the yard for four years. just put it on concrete this year when I enlarged the driveway. did it mostly to level the pup and I really disliked the bare dirt spot. (we would take extra time when parking the pup to make sure it was in the same spot so the dirt patch would not get bigger LOL). love that it is now level side to side. I am not sure how much moisture gets to the pup from underneath. it would sit there all winter with the rain, ice and snow on the dirt and did not see any problems each spring. my camper even smells new still. I think most water damage to a pup is from rain leaks when not keeping up with the caulking. we had to reseal around the vent fan in the second year due to some water leaking there.
HTT: "EscapeII" 2016 Shamrock 23WS (current)
PUP: "Escape" 2010 Rockwood HW 277 (gone)
TV: "Bruce" 2011 Dodge Durango V8 Hemi Crew
Just us gals (me, Sis and our daughters)
Traveled 19520 miles and 209 nights in 7 years and counting