Could it be your vent? The tank has a vent that outlets on the roof to aid in moving the smell up and away. If it gets clogged or somehow blocked, you could build pressure in the tank (not good). There should only be 3 holes in the tank; the roof vent, the toilet valve, and the gate valve at the dump outlet.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!