I just finished winterizing both of my 16,000 btu a/c & heat units. In order to create a positive pressure head for the circulation pump I cut up a general purpose spray bottle. Placing it over the open end of the sea strainer I could pour in the rv anti-freeze. A helper then started one of the units as I poured in the anti- freeze solution. I taped paper towel sections over the unit discharge thru hulls to show if the pink solution had circulated thru the condenser coils. it took about 1 1/2 gallons to do both of my units and about 30 minutes of my time.
There’s a commercial version of this funnel available for about $75, mine cost $1.78.