Don't put antifreeze in your FW tank, ever. It will take forever to get rid of.
You quite likely DO have a water heater bypass. You may have to remove a panel to find it, but it should be near the water heater. Even the most basic RVs have them.
Run the pump only on the cold water supply to keep its antifreeze out of the water heater, until you know the pump is flushed. You will use a lot of water getting the antifreeze flushed OUT of the water heater if you let it get in there.
Then put the WH on bypass and run out the hot water lines.
Finally take the WH off bypass, fill it, and run out the hot water lines.
THEN turn on the WH.
