OK first things first...Do Not use particle board as it will desintegrate when it gets damp. The heavy duty sawhorses are a good option as are the blocking and cribbing options, The one option that I have not seen is a couple of pieces of plywood slotted so that they form an X when assembled. Good Luck
Geez glad I checked back on this post...After reading some of the following post to this I feel that I need to clarify myself a little bit, First the X that I am talking about is used under the belly of the camper so when you pull out you simply assemble your plywood sheets on edge and set the camper down on it. Second, the plywood ,as Reddog stated, should be CDX and a minnimum of 5/8 inch thickness, also you should drill a small hole about the center of each sheet for a small rope to tie into so as not to allow the plywood X to spread too far out. Third Never Never Never try to block any jack supports over 12 inches high, even then you will be pushing the limits, if you have manual jacks and do not like cranking on them then either make or buy a drill adapter for that purpose. Good Luck