Here's a scary possibility. A leak in the tank itself, or at the fitting into it. If you're pouring stuff down the drain and the smell isn't going away, even temporarily, then there may be more to it than a gunky P-trap. You might try dropping the underbelly cover of the rig and have a look around down there, if you can.
The stink is coming from the P-trap. As mentioned before, disinfectants will kill the odor for a few hours at most. Trying to remove the P-trap to clean it is a major production. We took a good look under the sink. Keeping the smell producing slime from growing back would not be possible. This stink started about 5 days after we got here to FL. I noticed a faint bad smell from the sink before when we spent 2 weeks in this RV back in TN last fall. But it was nothing like now. We've been living in this RV since Jan 1st (snow birds until 4/1). This has to be a heavy slime/bacterial/mold in the down pipe and P-trap. Tonight before I go to bed I'm going to pour hot strong bleach water down the sink several times, then leave it overnight. The sink cover is absorbing this stench but strong bleach seems to have removed it... I just washed it thoroughly in hot soapy strong bleach water. As I also said above, this never happened in the other 3 rigs we had. An old Xplorer, an 1982 Sunline and a new 2009 Zinger. All of which we spent much time in. I looked under the rig today and there are no leaks anywhere.
Another possibility is dead rodents somewhere under the sink cabinet. A decaying mouse (or mouses) can smell pretty bad. Pull out the kitchen drawers, if so equipped, and look around for droppings or corpses.
The stench is, as mentioned above, coming from the sink drains. There is no smell under the sink or under the TT. It doesn't smell like a dead rodent. It smells like a rotten moldy dishrag, rotting moldy cloth or a foul kitchen sponge. As for rodents or insects... we've been lucky and never had an invasion of them in any of our rigs.
If we can't get control of this horrid smell without having to constantly manually remove and clean out the trap, I think we'll have to call Heartland again and see what they say. As far as I know this RV will run out of warranty the middle of next month. I'll let you all know how the hot soapy bleach works.
I do appreciate all the suggestions.