As we move into the older units, we tend to get far less info as things were more done by hand on paper and that means far less info online.
One of those things is the location and type of leveling jacks on your Rv! Sorry but this may take some looking.
One first question might be to ask if you do HAVE jacks as not all RV do and the earlier models are less likely than newer! So be sure you have jacks before looking for the tank!! Checking the owners manual it talks about how to level without mentioning how to use jacks??
If not on hand, this should be the manual for your RV and it has lots of good info:
Easy first step is if you have the big bag of info which should move with the RV. Do you have that bag and does it have the leveling jacks book that tells you about them?
If not you will need to move into more manual looking at things as we do not have what brand, model of jacks nor location drawings.
The thing to look for is a set of large hydraulic tubes from the jacks, running to one common control pump and reservoir. Black rubber looking lines about an inch thick that are strapped or run along the frame to the pump and pretty easy to spot. Good chance it might be just back of the front wheels but that is guessing!
Once you find the pump and tank, it will take some looking at where the tank fill is and what type. Look along the top or near the top for a plug and there will be one that is obvious. Whether it screws out or pulls out is a mix of brands, models, and types.
Hint? if you turn it and is seems to not come up and kind of sqeaks, it is likely to have a rubber type gasket and pull out!