Having done this before, it isn't too bad. There are two tanks running longitudinally in the dropped frame section. The galley tank is (usually) aft and running crosswise. The galley tank uses the smaller drain pipe, the black and shower tanks use a 3" drain pipe. My black tank is on the street side. But yours could be on the curb side - who knows what all Montana is capable of. Check in the passthru basement storage area and see where the toilet down pipe goes.
Both the gray and black tank valves ae at the rear part of the dropped frame. You only need to remove the screws holding the aft portion of the coroplast underbelly cover on the dropped portion of the frame to access them.
Flush the tank numerous times before doing anything. Drop the front end below level to prevent any leakage back through the opening once you remove the gate valve. The valve is easy to remove but difficult to re-install with the seals positioned properly. I cut the 3" pipe downstream of the "Y" and removed about 1" of pipe. This allows some slop to move the waste pipe and properly position the gate valve and the seals back into the tank fittings. Get a 3" No Hub plumbing fitting to bridge the cut drain pipe. This also allows some flexing once everything is back together and may avoid the tanks cracking somewhere down the road.
This is a common problem on all years of Montanas (and other brands). Between the inexpensive OEM valves, the tight fit of the plumbing which affects the seals and debris being caught where the valve blade closes that often results in a deformed seal, it is just a matter of time before they fail.
P.S. Much discussion and advice on this subject, and many, many topics can be found at the free to join Montana Owners Club forum. www.montanaowners.com
