There is a tank vent that exits through the roof. On our 23B it's right next to the bath vent. Possible it's stopped up? The flap on the toilet could also be worn (I think they are replaceable but don't know how - yet). Drain and rinse the tank(s). I find after I empty the black and use the wand (very unpleasant BTW) I can flush lots more "stuff" out of the black. The tank chems help but still don't make a basket of flowers. Unlike our houses everything that goes into the toilet stays with you until you empty the black - even then some "stuff" will remain.
I like to say that these things are like boats and airplanes. The bigger, the more complex. And there is a constant need to tinker, fix, adjust, replace, repair something.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks