Connecting a Dometic to an external 40 lb tank

You can buy "extend stay" and use it in reverse, but I don't trust those quick connectors on long run and prefer solid pipes.
Any plumber or advanced DIY will be able to make a Tee on the pipe going from the coach tank and connect it to external tank. Do it before pressure regulator and the connections can be small size.
I would also use copper tubing instead of rubber hoses.
3/8" tubing on high pressure side should give you plenty of flow to run even furnace.