I have had a similar problem with the black tank valve on my 2015 Montana 3402RL - stuck in open position after being difficult to operate for a while now. Though it is a work in progress, I have got it moving again with a combination of Thetford RV Drain Valve Lubricant and silicone spray on the exposed shaft. You might want to consider this before getting into the underbelly. Looking at the posts on this thread, this seems to be an underlying trend for recent Montanas with cable drain valves. Are you listening Keystone?
The drain valve lubricant, generously applied per label directions, was what mainly did the trick. I ordered it from Amazon and used a good bit freeing up that one valve. I believe the intended usage is routine smaller amounts for preventative maintenance. I now have ordered and received 2 more bottles and intend to keep periodically applying it.
