Toilet won't flush, really would like some info on what it could be.

I live in an rv. Our place is plumbed directly into the sewage lines from out of the tanks. When we flush the toilet, the water very rarely actually goes down - like there's too much water in the bowl. We have to plunge the toilet to lower the water and waste level, then flush it 2 or 3 times to get the water to be clear or clear-ish. Any ideas on what could be causing this?