Most likely the reservoir is in the rear passenger side corner. On mine it sits just inboard of the rear compartment (the one with the engine controls). It is a tall "can" that is filled from a port near the top. Using a small pump to get fluid up to the hole makes the task a lot less messy; I used a "drill bit" pump and some flexible PVC tubing. The instructions say to run one of the jacks down about 6 inches and then fill the reservoir.
However, if you didn't have a low fluid level before and you do now, don't rule out the possibility of a real leak. One of the hoses for our front slide sprung a leak and it was so slow that for a while we couldn't find it. I realize you probably don't have any slides, but one of the hoses for the jacks could be cracked and have a slow leak.