Sounds possible.
You could indeed use encoders, Stepper motors would be possible as well, you can command those very accurately how many degrees the motorshaft has to turn. Instead of encoder, You'll need home and end-switch to detect beginning and end of the leveler legs though.
I'd test a project like this a zillion times before I'd actually drive with it on a road. It would be nice if it could detect whether the camper is actually standing still as extra safety option. That may be possible by only allowing it to function when campers motor is not running.
I don't know how much force/torque is needed to operate the legs, they may be a bit slower with gearboxes, but it will probably save you money in expensive motors/electronics if you use them.
In case something goes wrong, I'd also think about an option to operate the legs by hand. You don't want the legs to decide to stay longer on the same spot. 