The filler plug is on the side, near the top of the reservoir tank. You will have to come up with a way to get fluid in there. When I rebuilt a jack and had to refill, I bought an inexpensive "fluid transfer pump" at an auto parts store. It screwed into an ATF bottle (in place of the cap) and had about an 18" output hose. It was just the thing. It worked great.
You should top it off with the jacks up, cycle the jacks (extend then retract) and add fluid again if it still needs some. If you have air in the system, you may have to cycle 3 times or so.
On our 1998 Southwind, I had to get totally under the coach to get to the reservoir. I put blocking under the front tires to get it up enough to have space to work under there.
Hope this helps.
Our "Cottage on Wheels" is a
1998 Southwind 32V on GM Chassis