I have an old Fleetwood Prowler and what you did is just fine. Drain the water then put antifreeze into fresh tank. Pump it through the entire system and then drain it. You can recover it for reuse next year. You lose a little each year but not much.
I need about 10 gallons to get the 12v pump to fill all the lines and the hot water heater without sucking air.
My fresh tank is 15 gallon so 2/3 full is plenty and I get the excess out of the fresh tank drain the same way I get the lines clear out of the low point drain.
If yours is like mine the low point drain is just a 1/2 inch plug and can be replaced with a hose bib. This allows you to add a short hose and makes draining the antifreeze back into the jugs easier.
If your unit is like mine you cannot access the pump to add a siphon or add a heater by-pass. There is just no way to access the inlet on the pump without removing the fresh water tank. Much easier to add antifreeze and reuse it.
