Bubbles?
Hi Imac:
Welcome to the Forum!
Have you tried using the Black Water Flush valve in the Docking Station. The Tank will have to be empty first, then hook up the fresh water hose to the flush valve and turn the water on. This may help to loosen anything that may be stuck in the tank and will check to see if your Vent tube is blocked as well.
From being at the Factory, the Vent for the toilet is behind the wall, behind the entry door closet.
Where are you getting bubbles at anyway? In the comode itself or other parts of the plumbing in the trailer?
Look forward to hearing back from you.
Also, if you have time, post a picture of your rig too!
Welcome aboard.
Talk to you soon.
Jeff