Our grey 1 tank is for our shower and vanity sink. I opened the valve(s) and used a leaf blower from the outlet side blowing back into the tank. I had the wife on the leaf blower while I was on the roof to make sure the vent was flowing. I could out that the vent on the roof is for both my grey 1 tank and my black tank. The black tank never had any issues flowing, so it was not the vent. I also made sure my valve gate for my grey was opening fully. Blowing air back into the tank seemed to help. Maybe something was in there restricting it, I dont know but it seems better now.
2017 North Point 375BHFS
2015 Silverado 3500HD Crew Dually Dmax 4x4