Yes, plumbing and leaks are frequent flyers here!
First thought is to find out FOR SURE that it is a waste tank leak as they are near impossible to pull and repair but they are also very close to bullet proof if we don't let them freeze,etc. But the pipes and onnections are more likely to be the problem. But before looking too hard for a grey or balck leak, it is always good to point out how bad plain old standing water can smell.
The water in the drain traps set for a long time and then if we drive and that water comes out, it can smell BAD!
First step for me is to test if it is old "fresh" water, grey water or black. You may have better info on that but do some thinking on alternate problems asno use to chase one to the death, only to find it is something else and be warned that water can be tricky!!
If it is a pretty consistent leak that comes out pretty quick, we can do more than if it is one of those that leaks out of a pipe and the water may stand on top of the tank and only run off when we move the RV.
Telling us more about what you are seeing/finding and when may lead to better ideas.
But if it is a drain leak, I want to find which for sure. Is it grey which can stink as bad as black or is it really black? One way to go is to drain both and add water to one or the other. Do it in a logical way so that when water does show up you can say which drain or stool, etc. makes it leak. Then when you do find water comming out, try to trace it back as far as you can see as that can eliminate some of the fittings and places which might leak.
Rodent damage can make it hard but most leaks are at connections, so let's pretend they have not done the dirty!
Some drawings of your RV are here:
Plumbing:
https://www.winnebago.com/Files/File...g37g_plumb.pdf
Parts with the water and sewage being down around page 75:
https://www.winnebago.com/Files/File...99/99ig37g.pdf
Good luck, hope for easy?? Taking the tanks out? Very difficult to impossible as they are "always" on top of the frame, axles, etc. !