See my thread on tyre issues in the tech forum.
Ideally you want to match wheel/tyre size to your tow vehicle, to give redundancy in spares. Also same track width is ideal, but you're pretty much stuck with what you have now anyway.
In practice, guard clearance was an issue for me going to same tyres as the 4wd, plus risk of causing your too-high tent problem, so sometimes you have to compromise. Just remember if you pick a tyre with a diameter any bigger than the current ones, your tent problem will be even worse.
I use this: (hope no one minds the link!) http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx
Cheers,
Cam