OK...we're sitting maybe 1200# or more on four places using milk crates? Sure the crates can handle 300# plus apiece but can your floor handle 300# in basically about a 14" x 14" area?
The whole point here is to distribute the weight of the camper over the ENTIRE floor and especially around the perimeter where the front, rear and side walls that carry most of the weight down to the bed of the truck is located.
Look at it this way, say you needed to fix the foundation of your house. Would you support the building using cribbing in the middle? Would you support the building using beams that extend from the perimeter walls on one side to the other under cribbing?
Remember, while a load bearing wall carries the weight of the house down to the foundation, if you jack up the house you need to support it in the same fashion. This isn't just an issue for older campers with possible rotted floors or sidewall/floor joints, it is something you should consider anytime you mount/dismount your camper. Leaving it on the jacks temporarily is how the mfgers intended you to use them. Leaving several hundred pounds of gear in there and then walking around in there puts unneeded strain on the floors and...on the sidewalls no matter what type of jacks you are using.
Just as most answers to the new owners who inquire about raising their camper to fit the truck cab/tub walls indicate you should consider something to distribute the weight around...be it plywood, dense foam or stringers...they usually mention that the choice should extend to the full width of the camper so as to support those perimeter joints as much as possible.
1) I would NEVER counsel someone to leave their camper on jacks alone in the off-season.
2) I would NEVER suggest they enter their camper when on jacks alone at all!
3) I would SUGGEST that in the off-season or during extended dismounted maintenance that they use a sheet of at least 5/8" plywood and then six milk crates or something around the perimeter would be OK...but it is critical to have a FLAT surface to set it on otherwise you defeat the purpose of even distribution of the weight.
4) If you dismount for the off-season, or any length of time, support the floor as above and I would suggest using the jacks with just a bit of lift to help distribute the load even more.
While a brand-new camper may not have any wood issues, older campers invariably do...so why tempt the hand of fate and put unneeded strain on the camper?
Just sayin'....
