That was a not uncommon problem in the older, horizontally mounted reservoirs. I recall post about replacing the seal but it was along time ago. You might try Searching on "pump seal" or "hydraulic pump leak" or variations. Look for posts pre-2008. Here is one that alludes to the seal change (about 1/2 way down). I seem to remember there was one with specifics about the seal needed. But a good hydraulics shop should be able figure out what you have and find a replacement.
http://www.montanaowners.com/forums/...ulic+pump+leak
Here is a better Post with replacement seal ideas.
http://www.montanaowners.com/forums/...reservoir+seal