The first winter we had our MH, I put the jacks down and wiped them with a cloth soaked in hydraulic fluid. After a couple of weeks, I noticed rust stains, so I polished them out and retracted the jacks. I now jack up once a month, high enough to get the wheels off the ground (one side at a time) and rotate the wheels a half-turn, so the tires don't develop a hard flat-spot.
I contacted the jack manufacturer about how close to salt water you could leave the jacks out. The reply was "We consider less than 60 miles to be close". Since we're across the street from the waterfront homes on a salt-water marina, we're probably less than 60 yards!