No worries on the name. Actually my first, middle and last names are all more common as sir names than as first so I have been called all kinds of stuff, just don't call me late for dinner!A thousand apologies concerning your name - and I promise not make the same mistake twice.
As to the link to the manual, yes access to entire manual may prove handy to have. For example - your putting the water heater in the by- pass circuit is quite correct on a Universal whereas that is verboten on other manufacturer motor cooling designs. This is because in the Universal the by-pass circuit is still open even if the main circuit is active - ie thermostat open. I confess surprise on that one.
However to the detail of that low mount diagram - your hookup may not be exactly the same. N.B. the petcock at the heater return line even in a low mount case. That is - even if the water tank is lower - Universal still advises a petcock at the heater thus to eliminate any possibility of persistent air pocket (albeit one not amounting to an air lock. ) Might try it. Putting one in is simple, cheap, and there is no downside.
Incidentally, I imagine if you take the space heater out of the calculus your motor operating temp will rise. This is - of course - a separate and parallel issue to creating hot water at the water heater.
Charles Erwin
When I replumb everything I will put a tee with capped line at the highest point below deck to give a secondary purge point. It will also give me a good point to attach my drill pump to circulate coolant for purging without the need to spill coolant while reconnecting the line. I am thinking that if I put a ball valve each side of the T then I can purge one side of the circuit and then the other.
The cabin heater might be causing the low heat issue but I would be surprise due to the overheating issue when the raw water HX was plugged up.