If your tank burps when flushed, you could have pyramid forming in your black tank. I would flush a couple of gallons of water mixed with a half a bottle of Dawn dish washing detergent down the toilet. Then fill the black tank up and leave it sit a couple of hours. Open the black valve and watch to see what comes out. Rinse and repeat if needed. To check your tank vent piping remove the roof cap and push a garden hose down the pipe and see if you can get to the tank. If you are really brave turn on the water and see if water comes out of the black tank drain. Be sure to have someone help you turn on the water and monitor the underbelly around the black tank to make sure that water isn't leaking from the vent to tank connection as they are sometimes not great. Good Luck!
2018 Redwood 3901WB pulled by 2016 Chev 3500 HD DRW. Goodyear G114 tires, Moryde I.S., Kodiak Disc Brakes, Splendide W/D, Tire Minder TPMS, Sleep # King.