What is a 351K? (Yeah I went and looked at your profile, but don't make people do that
So it's a Cedar Creek Silverback. Since it's a 2019 I'd let the dealer figure it out.
Do you have one or two vents on the roof?
The gray vents are most likely a wet vent so a clogged vent would cause that problem. If you have two vents try to figure out which is the gray tank vent and take the cap off and look down in it to see if you see anything clogging it up.
If you only have one vent do the same thing but realize that the gray vent is likely tee'd into the black tank vent so you probably won't be able to see anything unless the black vent is clogged too.