Paul,
Have you checked the ph level in your system antifreeze? If not, I would recommend that before you go to the work and expense of replacing what could be perfectly good coolant. You want a ph between 8.0 and 9.0 if you are running the recommended 50/50 mix. If your ph is between 7.0 and 8.0, you can add inhibitors to bring up the ph, but if it is below 7.0, you should replace it because it is too acidic.
I think that your 4 gallon estimate for the whole system with a Comfort Hot is too low because the Comfort Hot holds about 3 gallons itself.
As for what type of coolant to use, since your system is all double wall construction and cross contamination with your fresh water system is virtually impossible, ethylene glycol is the recommended coolant for all Hurricane heating systems in Beaver coaches.
However, in Beaver coaches with Aqua Hot/Hydra Hot systems the choice gets a little harder to determine. The earlier Aqua Hot manufactured systems all had double wall construction, and therefore they used ethylene glycol, but the latter models do not have double wall construction and therefore require propylene glycol (non toxic boiler fluid). Aqua Hot changed their system design in 2002, so if your coach is in the 2002 to 2003 range, you should check you system owners manual before adding any coolant, but normally 2002 coaches use ethylene glycol and 2003 coaches use propylene glycol, but if you are in doubt, always error on the side of caution and use propylene glycol.
Gerald
