Odor from shower drain

The water is probably just drying up right there. there is only a 1/2 cup or so in the trap to begin with. I would try to locate the actual leakpoint.. P traps themselves don't leak often. Should have a twist tighten joint on both sides.. and yes, that hole is what you have to work with/through. If the P trap does empty or evaporate away, you will get odor for sure. The answer of course is to have the P trap full of water..

At least one owner has increaded the size of the access hole, and used a larger grate to hide/cover the hole.

Hopefully there is no water leak, and all you're experiencing is the water evaporating from the trap.
You can pour a little RV anti-freeze (the stuff you would winterize the water system with) into each trap (shower, bathroom sink, and kitchen sink) it won't evaporate easily, and will stay longer.
ALSO you can place some plastic wrap over the filled drain hole to help with evaporation. You should do this on the toilet as well, don't let the toilet bowl dry out, then the flap seal can dry out and leak. Then it won't hold water very well.

Also, you should add some RV Black tank treatment to the gray tank after dumping, and also between uses. I have done this and it works as sometimes I am more than a month between uses in the Summer (I tend to go camping in Spring and Fall). It really helps with any stink, especially if you have it stored near your house and the tank vents are wide open and the wind is blowing towards your house windows.

Good luck.

Good suggestions all. I thank you.

I pulled the trap out (small hands finally work for something) and will reset it. Any ideas on a good sticky, non-hardening sealant to use around the shower drain? That was one source of the leak underneath.