Ok.. If an RV Gravity Toilet (Sea-Land for example) will not flush you have a very short suspect list.
1: Broken linkage and the flush gate/ball valve not opening.. Clearly NOT the issue here
2: Tank full....
3: Clogged plumbing (Well the plunger should have fixed that)
I have seen #3.. ONCE (well actually twice but same toilet)
From everything the OP tried it's down to Tank full
NOTE vent clogged will not stop toilet from flushing but it will.. Blow back at you
So your problem may be the tank is not emptying. I have not had that experience, but I have read about one case. Or two.
And again the suspect list is short
1: Valve not opening (Fairly common actually but you'd know that)
2: Clogged outlet .. The one I'm talking about... The workers when they cut the hole in the floor for the toilet pipe just dropped the wood down into the tankj. Where is eventually covered the outlet.. Wood was returned to CEO of comnpany.