You have a nasty problem and , it would appear that the tank has a pin hole leak, and possibly some mould that really starts to smell when wet.
Depending on the brand of tank the foam insulation could be removable with the tank out of the RV. Water heater removal on most RVs is not a big job, and removing it could very well be your next step to check and possibly repair your problem.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.