Since the bowl holds water, it doesn't sound like the odor is coming up through the toilet bowl itself, but it could be coming through the overflow tube
Most RV toilets have some means of a overflow drain which is usually a small tube that runs from the top of the bowl area (usually near where the water enters the bowl) directly to the black tank. This tube allows the water to enter the black tank rather than overflow the bowl if the fill valve should leak and overfill the bowl.
Usually some water is supposed to go down this overflow tube whenever the toilet is flushed which keeps some water in the tube and prevents odor from traveling up through it.
Try looking for that tube and pouring water directly into it. It's possible that the tube is empty and you are getting odor throu it.
Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV