The holidays distracted me from keeping ya'll up-to-date with my solution, such that it is. Sorry.
Thanks to Scooter Ride for reviving the thread.
After thinking things through, I decided to make my connection on the High side (between tank and regulator). It eliminated some complexity (another regulator) and made me buy high pressure hose instead of low pressure (I can use the high on the low side if I decide to go there, but not the reverse.
I looped a 2' piece of 3/8" copper, flared the ends and used it to insert a "T" between the tank and regulator. The copper is a temporary solution. If it woks out, I'll replace it with a flexible line. In the mean time, it is sheathed with two layers of that black plastic "snakeskin" stuff commonly used to contain wiring. I attached a 12' flex line from the "T" to the area where I want to hook the aux tank and installed a shut-off valve then an ACME nut, to the hose.
All seems well after fixing 2 leaks. But I'm still not comfortable.
Scooter Ride, when I began I did consider what you suggested. As I understand things, the "Fill" port on my tank is for liquid propane and the "Outlet" port is for gaseous. A 20# bottle has the valve that serves both purposes. While I think it would work, again, as I saw things, the best I could hope for was for the 20# bottle to deplete at the same rate as the MH tank since mainly gaseous propane would be introduced to the system. This really makes the 20# part of the MH tank. I'm looking to use the 20# instead of the tank. Make no mistake, I could be completely wrong here, but it seemed to make sense at the time.
38 Special, I would LOVE to have done what you did. Alas, my tank is between the chassis rails and access is the problem. Everything has to be done with remote control in mind. My skinny neck doesn't like gravity working on my fat head in anything other than the standing position, so my goal is to stay out from underneath the MH as much as I can.
As always, the upside of any quest is learning a few things. But, they're not always helpful things.
What I have done so far is not terribly elegant, but it does work. I'm still considering moving everything to the low pressure side and buy another regulator. Still trying to understand the virtues of a two stage regulator over a single, so if anyone can make me smarter on this or anything else, PLEASE DO.
As always, apologies for the length of the post.
Thanks to all,
Steve