parked on 12 inch planks keeps the tires out of standing water which is a killer of tires- dry rot...
several manufactures stress that keeping it level with a tilt to the front or the rear for water run off the roofline is best. they also stress not to lower the stabilizer jacks if you are in an area where frost is an issue. this will twist the frame as well Last year we had 9 feet of frost in the ground where we live.
Concrete will also permit puddles of standing water.
several thousand pounds sitting in harsh weather conditions for months will cause flat spots. (our truck sits for weeks and apparently even radial tires can flat spot like old bias ply tires did)
2009 Heartland Sundance 31.5 XLT bunkhouse
2013 Ford F150 XTR ECOBOOST
"Of the troops - For the troops "