Install some metal brackets on each corner if it doesn't have them, and go with a jack on all four corners. You won't regret it ... you might have $1300 in a three jack set-up, or about $1450 in a four jack set up. But if the truck tips -and it probably will on three-points, then you'll have a damaged three jack camper that will cost more than the extra $150 or so for the fourth jack.
PLUS, lower and raise will go a lot easier as well. There are lots of ways to save money fixing up an older model camper -- This is NOT one of them.