I've had some similar experience with previous 5ers. In one it was the gray tank getting full but that's not your issue. In another we actually had a poopoo pyriamid from the previous owner that needed to be cleaned out. We had a short trip coming up so I put about 15 gallons in the black tank and a couple of the TST Max Drop ins (these are what I keep on hand to keep our black tank smelling citrusy fresh). I let the solution slosh around on the short trip to the campground. At the end of the weekend I emptied the tanks and the problem was solved.
Kind of a long story to ask if you're sure your black tank is clean.
TST Max here. These seem to work well to break it all down:
https://www.amazon.com/dp/B000BUQOEK?ref_=ams_ad_dp_ttl
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Jeff & Sandi (and Teddy - 7lb Schnorkie)
2018 Montana HC 305RL / HW Progressive EMS
2015 RAM 3500 Laramie Longhorn Crew Cab 4x4 DRW / Demco Recon Hitch on RAM Puck Ball