Just thinking out loud here, but one reason the front bunk might smell worse or different than anything else in the trailer is that often people remove them to storage in order to have more room in the front of the trailer. And maybe it was not "good" storage...
If it were me, I would first remove the bunk from the trailer. That's six screws and about five minutes (two hinges, three screws each, on top of the mini-shelf it's mounted to).
Next I might
try leaving it to air on a sunny, windy day (like you are going to get any of those any time soon!).
But really, I would probably just remove the foam (there is a zipper on the long straight side on my bunk cushion) and wash it or replace it with new foam. You can do the "grape stomp" in a bathtub or on a tarp on the lawn; or, I have heard Robin G. say that she washed hers in a machine (I would guess a non-agitator machine, but I don't remember).
Once you have the foam out, you should be able to see how to disassemble the rest of the bunk cushion. I would guess there's a wooden stiffener somehow attached to the fabric. If you could separate those two, you could wash the fabric.
You might get lucky with just washing the foam, but ... wouldn't it be nice to know the fabric was clean too? Then it could just be your