Grey tank stinks really badly

(Third time the charm? I keep getting errors. Edit: OK, this asinine, it wouldn't post the first two times b/c the original title was: "Grey tank smells like feces".)

We bought a used unit less than 6 months ago. After we had been in it a while I figured out that the previous owner had not taken very good care of it and I've had to do a lot of small corrective maintenance. So this may be an issue that developed over time and started before we got the unit. We did not have this problem when we first got in to it. So, maybe it's my fault, IDK? All I can say is I've done everything I can think of to maintain my systems and one of the things I had to do was clean a stalagmite out of the black tank.

I've got a broken neck and movement is very limited, so is hand strength, lifting capacity and the ability to crab up under stuff. I'm also full-timing in a park where my wife is work camping enough to cover both of our hours and they do not allow major maintenance. So suggests like "drop the grey tank and gut half the plumbing" won't help.

FYI, I keep my grey valve closed and treat it like a holding tank. Please don't give me a bunch a flack about it. (People on the web tend to get nuts over stuff like that. I really don't need to hear it.) After a lot of reading and my own experiences I decide to do it this way. In this unit, for reasons that I don't understand, the traps do not function as I expect and smells can get from the grey tank in to the living space. Now granted, I'm not a plumber, or even all that handy, but I thought the whole point of a trap was to prevent out-gassing? (So I really don't even understand why I'm having this problem at all. Shouldn't the traps prevent the stink from getting out of the tank and in to the living space.) I've found that in this unit the out-gassing problem is way, way worse when the grey valve is open. With the grey valve open when we run water through a trap I can only compare the escaping gasses to something really disgusting like maybe taking a straight hit off a sewer pipe bong. Before the tank itself started stinking keeping the valve closed prevented sewer gasses from entering the living space. Now I'm danged if I do and danged if I don't. Open it and I get the out-gassing when the water is running. Close it and I get the out-gassing when the water is not running! But, the stink coming off the tank while pretty bad is not quite as bad as the stuff coming out the sewer with the valve open.

The problem with the tank stinking has only developed in the last few weeks and just keeps getting worse. When we first started closing the grey tank valve to prevent out-gassing while running water I was using a mix of bleach crystals, Calgon, laundry pods and black tank pods to control odor / growth in the grey tank. It worked, but we were going through a lot of money running so many chemicals through the tank b/c we end up dumping the tank about every other day. So I changed things up trying to save money and used less chemicals, but never stopped treating the tank entirely. Although I did quit adding bleach as the park asks us not to due to them employing a leach field.

I'm at my wits end here. I've tried everything I can think of. I've gone back to regularly using a lot of chemicals to treat the tank, even though it's expensive. I even used a 4x shot of bleach (enough to treat 140 gallons) and even that did not kill the smell.

I've got a jug of actual “grey tank deodorizer” on the way from Amazon.

As stated: I am no plumber at all. And b/c of my neck I've been reluctant to start ripping on the plumbing. But the problem is getting bad enough that it's time to start looking at that. The traps look OK? I mean they look like every other trap I've even seen: a pipe with a U bend in it. Could it be that they are not deep enough? Should I look at maybe swapping them out for a deeper trap? I can't get to the one under the tub. But the wife should be able to swap the ones under the sink.

The kitchen sink is the worst offender, although all three drains do it. Would cleaning the kitchen trap help? Should I maybe look at some kind of drain screens to catch food when we're washing the dishes?

I'm also worried that there maybe some cross contamination between the black and grey tanks. Ever since we bought the unit it looks to me like the plunger on the black valve will not seat completely in to its socket; meaning when the valve is “closed” there is still about 1/4” of the push rod exposed. My RV experience is very limited. But, it seems like most of the ones I've looked at expose none of the push rod when closed fully. I've done everything short of hitting it with a hammer. It is as closed as it is going to get. But, I left some as yet unused water in it the black tank and no water in the grey when we moved the unit to test my theory about it not sealing all the way. When I pulled the cap off the end of the pipe I got a big gush of water. So, obviously the black valve is not holding in water in transit and AFAIK it should. After all, a lot of black tank cleaning methods tell you to do some travelling with a little water and some cleaner in the tank. (I did that in our old unit and did not get a big gush when I pulled the end cap.) I'm wondering if I've got black water leaking and getting in to the grey tank since I use it as a holding tank. I'm not able to, nor in a place where I can, drop the black valve to replace it. Is there anything other than hitting it with a hammer that I can do to try to get it to close more fully?

Any help appreciated.

Thanks.