The tank leak discussed in the OP occurs between the white grommet installed inside the tank and another white one the black pipe is inserted in. During construction, the two grommets were pushed one inside the other at the tank wall, the tank outlet. The cement between the two grommets has failed. There is no leakage from the black pipe. I assume the stress of bouncing down the road and operation of the black tank valve are the causes. If you push on the black pipe you can open the gap between the grommets and increase the flow. Otherwise, it's a annoying, smelly drip.
The juncture of the two grommets is raised about 3/8" above the level of the black pipe and the space between the terminal lip on the inside grommet and flange edge on the outside grommet is about 1/8". This presents a rather narrow surface to seal. Once I get it sealed I have to secure the black pipe in such a way to avoid repeating the damage.
Not knowing the composition of the white plastic grommets is causing a problem in knowing what product(s) to use. Any suggestions?