We just bought a small Dometic (smallest one at Canadian Tire) and it leaked the first time we used it (eww!!) We aren't 100% positive why, but it was either a.) too full and got bumped and it spilled over somehow where the flush valve is (my thought) or b.) the emptying cap wasn't properly tightened or got bumped and loosened (hubby's thought).
We took it apart (emptied & cleaned it) & out it back together assuring the emptying lid was super tight & that the two side latches were properly latched. We also assured it didn't get too full before we emptied it. This isn't specific, but it worked for us, no more leaks
. I think ours is the smallest model available, if yours is too, they seem to fill up quickly even if only for nighttime use (for the wife), so we emptied every two days.