I have the water heater and the cabin heater connected in series. The water heater sits about 2 feet higher than then engine. The cabin heater is lower but still the heater coil is a high spot. The procedure is to fill the engine with coolant and run for a couple of minutes, then refill. It takes a couple of cycles to get the air out but eventually it is all gone. Over the next few hot-cold engine cycles the expansion tank also requires a top-up.

DO NOT USE PLASTIC ANYTHING if you are going to put a bleed fitting in. Copper or brass only.