Maggots can be a nasty surprise where ever you find them. Since they are usually the larvae of flies.....I would check to see if flies can gain entrance to the black tank through the vent line on the roof ......or any other way. So stop the flies gaining entrance to stop new maggots; and flush the black tank top to bottom to get rid of those already in the tank. Maybe try an RV style toilet wand with the rotating head which attaches to a garden hose (use an anti backflow valve on the hose where it source attaches outside the coach).
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