Idk the particular recommendations for that camper, but surely, you've accessed the inside of the camper while it was off the truck and sitting on jacks?
How did it feel? Stability?
Anything sagging underneath?
How well do the 20+ year old jacks work if you cycle them full height a bunch of times?
90% chance it's perfectly good with some simple support under the belly to help stabilize or take part of the load and might be fine with no additional support.
Someone with a similar model will likely reply now that we know what camper you're asking about.