Help! One of my jacks won't retract or extend!

Camco EAZ Lifts are rated at 5,000 lbs. per pair, so yes, you should be fine using those for temporary support, but as noted above, using a wider board (i.e. 2x12) would help distribute the weight. Reico-Titan replacements would be my recommendation, but be aware that the gearing ( rate of raising/lowering) may be different than your existing Happy jacks. It would mean you’d have to toggle between the two different brands to play catch up to keep the camper level.