[Wally Arntzen]

Brian,
On my 88 coach I have a check valve where the line enters the tank on all of my tanks. I think yours is similar to mine and has two tanks stacked on top of one another in the front, one in the rear and a small tank for you toilet if you have an air operated toilet. My toilet tank was located in the wheel on the drivers side and I relocated it in the bay that contains the water heater. I also installed a check valve on the small tank.
I also have a small water seperator installed after the 110 air pump, and at the entrance side of the toilet air tankj.
I live in my coach and when I,m parked here in Minnesota in the summer the water seperator at the 110 air pump gets completely full every week and push the dump valve at the bottom to drain it. The one at the toilet gets very little moisture in it because the air to that tank comes from one of the large tanks and is already filtered before it gets to it.

Tuga is right about draining the tanks. I do all of mine every 6 monthes and it is suprising how much water is in them even with the filtering that I do.

I have changed my engine air seperator probably similar to Tuga,s which has a spin off filter on top that I change every year.

I use my coach in cold weather so I have a quart of alcohol put into each of my large tanks every october to prevent air line freeze ups. This gets drained out when I drain my tanks in Arizona in December.

Excessive moisture over time will ruin the check valves especially in old coaches like you and I have.

If you don't have them at your tanks I suggest you put them in.
MAKE SURE YOU PUT PROTECTION UNDER THE COACH BEFORE YOU GET UNDER AND WORK ON THE AIR SYSTEM.
Good luck my friend, Wally