Posters above have hit all around the issue at hand, that is you can probably never drain or blow all of the water out of an Aquahot. The problem is, there is not a tank
instead the water is stored in coils of copper pipe. People have tried
to winterize by blowing the water out only to find some water remained, and that
water froze bursting the coil $$$$$. The manual above is not for your Aquahot,
yours is probably an AHE-100-02S.
So here's hoping that it has not already frozen.
If the heat ( Aquahot ) has been running you are probably ok.
If your water flow seems good you need to winterize.
Buy RV Camco anti-freeze and make sure the Aquahot has anti-freeze.