This has been a great source of frustration for us also over the years with different campers. We now have a low pressure, quick-connect line from the 20lb tanks at the back of our camper.
Almost all BBQs already have the built in pressure regulator, so if you could even cobble the fittings together you would be doubly lowering the tank pressure, which wouldn't work so well.
I have heard that the small Weber grills Q100, Q200 can easily have their regulators removed so you can use the existing low-pressure line. You'd still have to buy an adapter and these Webers are HEAVY and bulky. Something like this:
http://propanegear.com/weber-q-rv-quick-connect-kit/ or something similar on the usual shopping sites.
We finally gave up on grills and their mess and got a CampChef Ranger II burner for cooking outside with a large griddle.