help! 2003 winnebago possible rear differential seal leak?

hello all, i took our 2003 adventurer 35u to get gas the other day and just noticed a small streak of what appears to be clean differential fluid
on the dual wheel chrome rim, (smells like rear end grease). i popped the center cap off and the hub with a number of bolts on it appears to be wet, but not "dripping" wet, i got under the rv with a light and could not find any other signs of leakage anywhere on, behind, or between the wheels in question, it is bone dry. i then removed the filler plug and the fluid level is about a 1/4" below the plug hole and appears to be pretty clean. after reading some info about the axle, i checked the vent plug on top of the differential on the passenger side and it was loose (hand tight) and kind of dirty with caked grease, but did not appear to be plugged up and the cap on top moves freely. so i unscrewed it and took it out for cleaning but it appears to be working as the cap is free to move up and down as well as rotate. has anyone had experience with this type of problem? is it most likely the seal that needs to be replaced and if so, any idea on what it's going to cost at a truck garage?
after doing more research, i am thinking now that it is the outer axle flange gasket that is most likely the culprit, so i will attempt to pull the axle on that side tomorrow and use silicone to replace the mylar gasket that appears to be broken! has anyone on here done this procedure before? i am use to working on cars and have not worked on a truck chassis before now! lol

I really can't help you, but I can bump this back to the top of the board by responding.  Hope somebody can help.

  Look for a filler plug on the axle hub before you pull the axle. They?re usually the culprit. Usually 3/8 or 1/2 inch pipe plug with Allen head. You may or may not have the plugs, depending on duty rating of axle.
  It?s a straight forward job to remove axle and replace gasket or reseal with silicone.

thank's, i've decided to pull the axle and use silicone as it appears to be the outer plastic gasket is brittle and cracking allowing fluid past it.

well, i managed to get the wheel simulators off, removed the 8 axle flange nuts and was able to used a puller to slide the axle shaft out about an inch (i was afraid to pull it all the way out for fear of possibly not being able to get it back in!) which was not an easy task. the very thin plastic flange gasket was brittle and in pieces, so i scraped and picked it all out and cleaned it with brake cleaner, then put a bead of rtv silicone around the flange surface, let it set up for an hour, then replaced the nuts and drew the axle back in by tightening them. i still am not sure what the proper torque is for the flange nuts though after searching and searching for the correct numbers. the closest i came to getting a straight answer was from the dana spicer manual which says they should be torqued to over 200 ft/lbs which seems like an awful lot to me and is based on the size of the studs i think, not the 7\8" nuts? still confused and will try and call around today to try and pin down a number!
don' understand why it is so difficult to get the proper torque specs for something this important and common!