With the air bubble you mention, I would suggest it is a venting issue. Normally this happens when the tank is almost full and there may be a air blockage at that point. You may want to remove a vent on the roof and squirt some water down it to possibly remove anything that might be blocking the vent pipe. A product we also have begun to use in our tank is call RV-Digest it. It's made by a company call Unique and can be purchased right from the mfg. Good luck hope you get this resolved.
Steve & Sally / HiTee & Hudson Our Little Poms / Heidi & Houston Forever in our Hearts
04 NEWMAR MACA 3778 W22 / 05 PT Vert
Michigan (Summer) Florida (Winter)