The first thing you need to do is to split the line where you want to install your T so you can see what you need. If you pull the pipe on the upstream end of your regulator, it's probably a 1/2" pipe thread. If you pull the connection from between your propane tank shutoff valve and the pipe to the regulator, it MAY be a pipe thread or it may be a propane spud or even an inverted flare fitting. It all depends on how your manufacturer put it together. Once you get it split you'll be able to tell. All possible fittings should be available in a good hardware store, auto supply store or at a propane dealer.
We've done what you want to do, but mine is set up to feed propane from a 5 gal BBQ type bottle to the rig so I don't have to pack the rig up to go into town when I need propane. Just be sure you put a propane valve just downstream of your new T to the BBQ. That way if your hose springs a leak, it can be isolated from your rig propane system.