Thanks to all responses. We had the same issue this year with our 2018 375FL. Was able to get some antifreeze thru all faucets, but not enough to suit us. Bought a separate AC operated pump as a winterizing back up for next year, but will try out suggestions in this thread for the RV pump. If all approaches fail, the back up will be to use the alternate pump, putting antifreeze theough the city water inlet (we are on city water 100%). I suspect the check valve is the culprit, and I wish I’d read this thread earlier!
Greg & Mary Campbell
2018 Montana High Country HM375FL