Don, I have ben motorhoming since the late 70's and have never replaced a sewer valve. Maybe tomorrow mine will go bad, but hoping the preventive steps will ensure this is one less problem on the road.
Sorry about not keeping up with the topic, but here goes.
Use a 1/4-28, short Zerk fitting.I used the short thread because of the thin plastic. The Tap is a 1/4-28 and bit size is a number 3. After drilling the hole, give the tap only a couple of turns, so as the zerk seats properly and snugs up. Prior to turning the zerk in, apply som silcone, caulk or some other tghread material to insure you never have any leaks.
The Grill Miester ask about grease? I used the same grease I grease my coach with., I don't think it would matter as the longevity of the valve goes, but if you live in a very cold climate ( I am in Florida, and only travel to cold weather places), you might consider some sort of lighter grease, but now sure about that. My best suggestion there is to move if you can. Just joking, don't move down here, move to southern California with the Grill Master. I here Redondo Beach is nice this time of year.
Sorry, one time, Gary (grill miester) ask about which side to put the thing on. I have done it both ways. It all depends on which is easiest to install and get to the zerks. My present coach the valves turn down so I put it on the side, instead of the end. I have done it both ways. But put the fittings close to the side of the plunging rod as to get some grease on the rod as well. I hope this will help.
The pictures Gary posted will help clarify what I am trying say.
I was concerned about grease deteriorating the rubber seals, but they are not rubber some other material that seems to not be bothered by the petroleum grease. If you use the GEO method to clean your tanks, you will want the Zerk's to re lube the valves, as the detergent removes all lube from the valves.