I think that blw2 is on the right track in that it is likely the vent tube from fill port to the fresh water tank causing the fill burping problem. On my 2013 ACE 30.1 the vent tube is probably positioned below the main fill tube somewhere so occasionally the vent tube can get water inside when filling the fresh tank. When the vent tube no longer passes air, the fill water starts to burp back out. I use a small piece of rubber hose sized to fit over the vent tube hole and blow the water out of the vent tube back into the fresh tank......which then restores the proper fill tube water flow. I would like to check the vent tube positioning to fix the issue, but it is practically impossible to get access.
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