My Bluebird Wanderlodge weighs north of 20 tons (40K lbs). I just moved to a new house and we put in a pad to park it on. We did 6" of concrete with about 6' of rebar reinforcement on either end of the pad and wire mesh throughout the whole thing. The contractor also used 5000 PSI concrete.
I've been parking the Bird there for about 5 months (after waiting about 7-8 weeks before using it) and so far no cracks. I do recognize that its only a matter of time before it does crack (concrete always does). Also, I watered the concrete daily (or took advantage of rain) after he poured for about 2 weeks. That is supposed to help the strength greatly.