Gravel = no problem.
Pea gravel for a driveway is another story, but not Germaine to the conversation.
Put some pads or patio blocks under jacks.
Unload camper, mostly done at this point.
I used to support the camper floor and take weight off the jacks. Quit doing it years ago when I realized it wasn't necessary for my camper.
You can stabilize the camper for use off the truck a couple ways. Or combinations of methods.
1. Lower it down to reduce stress and sway on legs.
2. Support camper floor.
Many mthods for #2. Conger blocks and boards. Sawhorses and boards or plywood. X brace like Scooby mentioned. Pallets, barrels, etc etc.