My hot water heater is above the engine and to solve the fill problem I put "tees" on both engine coolant hoses that I can access from on deck in the cockpit lockers. The water flow is through the "tee" as usual. There is one leg coming up with a short length of pipe and a cap. I use these to fill the last bit and remove any bubble. I leave the engine off, remove both caps and with a funnel pour anti-freeze mix into the other until there is over flow. Then cap and run for a bit, shut down and check. This usually gets rid of the bubble, the expansion tank can now adjust for hot or cold engine.