I still have the HydroHot manuals from when we owned the '04 Monterey. I thought I put them in the BAC files system years back. If not there I can email them in .pdf form.
This is a paste from the bleeding page.
Bleeding the Interior / Fresh Water Tank Heating System
NOTES:
A. It is recommended to use either an Ethylene-Gylcol or Propylene-
Gylcol based Antifreeze for all Hydro-Hot installations.
B. Be sure to remove the Hydro-Hot's "Access Cover" before beginning
this procedure, see Figure 2.
1. Remove the Hydro-Hot’s Pressure Cap and completely fill the Heat Tank
by pouring in 4 Gallons of a 2/3-thirds Water and 1/3-third Antifreeze
Solution, see Figure 2.
2. Replace and tighten the Hydro-Hot's Pressure Cap.
3. Locate and disconnect the two Yellow colored wires located inside the
Access Cover Area, see Figure 32 for wire details.
4. Connect the two Yellow colored wires together by utilizing an ATO type
automotive fuse, or similar, see Figure 32.
5. Switch ON any Interior Zone Thermostat to its maximum temperature
setting. Allow the Circulation Pump to operate for approximately one
minute and then switch it OFF. This will activate one of the Hydro-Hot's
Circulation Pumps which will fill the heating system's plumbing lines.
6. Again, remove the Hydro Hot's Pressure Cap and completely fill the
Heat Tank with additional water / antifreeze solution.
7. Repeat steps 5 and 6 for both Heating Loops until all air has been
completely bled from the entire heating system.
8. Remove the fuse, or similar, from the two Yellow wires and reconnect the
wires back to their original wiring configuration, see Figure 32.
NOTE: The Yellow wires are not polarity sensitive, so either Yellow wire
can connect to either Black wire.