You can piggy back as many tanks as you need but remember, that BBQ line comes directly from the main gas manifold which is behind the main tank regulator of the factory system. If you connect another tank to the BBQ line it MUST have it's own regulator inline or you will over pressure the factory system. This will cause problems (and prolly a fire) with furnace, stove, water heater and any other propane items in the coach.
I would sit the extra tanks near the BBQ connecter. They must have pressure regulator on them. From there you would be feeding the extra propane into the manifold at correct pressure and would have no problems.
Also remember the tanks sitting on ground are a safety concern so you need to put some protection around them so they can not be ran over or bumped into accidently.