How much clearance do you recommend when parking?

Hmmmm - not so sure about that bush in the back. I'd still use a line level. Lumber can be warped. I wouldn't be concerned about the slope. Doesn't look much worse than mine - but you will have fun with your WDH, hooking more than unhooking. I'm also guessing no extending slide outs too. The kids might have a better time at a CG.