To come back and update this thread...
I was able to get it all hooked up, but for future finders of this thread and conundrum I ran into, my 2006 Vectra 40FD actually had 3/8 hose from the valve on the propane tank to 1/4 MIP into the regulator.
To get the new/old propane connectors on the Camco Extend-a-stay to connect, here are the parts used:
On the Extend-a-stay inlet side: (LASCO 17-5381 Male QCC-1 by 1/4-Inch Male Pipe Thread Brass Adapter) -
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Plus a 1/4-in Threaded Coupling Fitting to connect the male to male 1/4 MIPs.
On the Extend-a-stay outlet side: (Mr. Heater 1/4-Inch Male Pipe Thread by Restricted Flow Soft Nose P.O.L fitting) -
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Then, as mentioned before, the 12 foot pigtail to connect the external propane tank: (Camco 59035 12' Propane Supply Hose)
https://www.amazon.com/Camco-59035-P...4143245&sr=8-7
FWIW, I don't get any referral credit or kickbacks from Amazon for these...just where I got them