Fork oil is the correct answer. The OP said they didnt see any leaks either. Here is a link to the Lippert document that has the procedure for doing it, along with which brands are approved.
Lippert I had the popping noise, especially with temperature changes. Sounds like a really loud acorn hitting the roof. I performed this procedure, and the problem went away. the procedure has you raise all the legs, remove a quart of the hydraulic fluid from the reservoir, pour half the replacement fluid in, lower all the legs, and then pour the remaining fluid in.
EDIT: I reread the OP post, and understand the LEAK. Sounds like fluid is getting past the piston internally from the pressurized side to the non-pressurized side. If you are mechanically inclined, I'd do the fork oil procedure first, and run the legs in/out a few times to circulate the fork oil and see what happens.