[Jackflash]

I think I'm watertight now. Thanks to everyone for their input, especially  you, Rudy, for your experienced advice and swinging by to takeep a gander at it.

[Richard]

Would you mind sharing what you did?

[Jackflash]

Not at all, sorry, should have included that on my last post. Basically the steps were as follows:
1. Disconnected the burner and moved it out of the way, providing easier (not easy) access.
2. Broke the incoming engine coolant line at the union between the pump and the tank.
3. Used a torch to break the sweated joint where the elbow connects to the line just outside of the tank.
4. Removed the elbow from the short section of line I pulled out.
5. Filed both exposed ends of the line smooth, removing as much of the remaining solder as possible.
6. Installed a Shark-Bite elbow and reassembled the unit.
7. Pressure tested the repaired circuit, looked good.
8. Fired up the engine and diesel side of the Aqua-Hot and let them come up to temp. No leaks, Hallelujah!

I think I'm fixed on that one. The fittings for the sewer dump valve pneumatic cylinder came in today, so on to that.