Greetings from sorta near the Track too Tough to Tame, AKA The Lady in Black, S.C. For the non NASCAR crowd that's Darlington and today is RACE DAY! Nope, I'm not going.
The grey stuff looks like Butyl sealant. I just used some last year on tank valve R&R. Not sure about the black, Silicone RTV??? I had a time finding that stuff, found it at an RV store as skylight sealant. It stays flexible a bit like used chewing gum.
But how about a slow fill test with clean water of the tank and see if it's an actual issue or left over from mfgr of unit?
Access to the no fuss flush likely means time to find it's general location then make an access hatch in the pan keeping in mind might as well make it big enough for valve service if ever needed.
2005 Dodge Ram 2500 5.9L 6 Speed
16" Michelins, Hi Spec Wheels, Max Brake, Dexter 4 Piston Disc Brakes, Carslile Actuator, Equal-I-Zer, Dill TPMS. Campfire cook. BMV-712