Dextron, and there are several numbers, not all mix well, is automatic transmission fluid used in many automotive transmission and in Power Gear Jacks.
I see folks have yammered about HWH jacks a lot in this thread, I see one poster said that HWH makes or rather sells it's own brand
POWER GEAR, on the other hand, recommends Dextron, I forget which number and check the sticker when I go to get some (NOTE The number is a roman numeral and it can fake you out)
Thus, I will assume you have POWER GEAR jacks
It is kind of chilly out just now
Power gear jacks use a float valve in the fluid tank, a full tank = retracted jacks, alas, when it is hot out that fluid expands faster than the tank some overflow may occure
Then when it gets cold (It was freezing in Wisconsin this AM) the fluid "retracts" and no longer fills the chamber.
From the tone of your post, it appears you know you need to add fluid.
So, where to get it:
Any auto parts store, Truck Stop, most gas stations, Sears, K-Mart, and other mass merchindisers with auto departmetns. Pep Boys, Camping World, your RV dealer, Many if not all hardware stores, Many drug stores like CVS or Rite Aid or Wallgreens, Many supermarkets (The auto isle in those cases) .. In short, dang near anywhere. Comes in a 1 quart bottle that looks exactly like an oil bottle, in auto stores and departmetns it may well share a shelf with motor oil. Do not confuse them. Looks something like the photo (NOTE: Label may be different but WILL say Dextron on it)