I vote for the board idea, I've seen some foundation repair outfits that use "tons" of three quarter inch sheet plywood to drive their equipment on. I guess it would all depend on how soft your soil is there.
I remember years ago one of my co-workers came back from a trip with his MH and didn't realize how much rain we received while he was away, he buried the MH up to the axels. Ended up having a HUGE wrecker come and winch him out. You should've seen his front yard, turned into a long landscaping project to fix up the deep ruts. He laughs about it now but it wasn't so funny that Sunday he got back from his trip!
Dan