There really isn't a "diagram" for the propane...unfortunately, the LP system is assembled just like the electrics; a general idea, open to a lot of "interpretation" by whatever team is doing the assembly that day.
As a rule of thumb, the two tanks will attach, one each side, of the auto changeover regulator. From the regulator (most being two stage), the LP will continue by an LP hose down to the frame of the RV. For safety purposes, most all of these "main lines" will be black iron pipe, mounted external to the trailer (under the floor and protected on the inside face of the frame). This black iron pipe will run as far back as necessary before the individual copper lines branch of it to go to the various LP devices in the RV. Most RVs will have those appliances in close proximity to each other to minimize the number and length of flexible copper pipes that have to be run. Since each trailer is slightly different in layout, that is about all the "diagram" you'll ever find.
I have seen a "diagram", but, it is next to useless and far from accurate as it shows each individual appliance with its own branch from the main black iron pipe, and I know for a fact that many RVs will have a copper branch line that terminates in "T" with several appliances fed by that branch line.
About all you can do is to visually follow the lines from the tanks and test each joint that you come to as profdant139 recommended; no looking at a diagram and having an "ah-ha" moment, unfortunately.
