If you wanted to have something like this "professionally engineered" where or who would be qualified and who would actually be willing and able to perform the work? An RV dealership? Camping World?
I asked a propane supply outfit for something similar. Wanted to add a shutoff valve to my T connection between the main tank valve and the regulator, so I could turn off the gas to the grill line, without shutting off the gas to the regulator/coach. I asked a major Michigan propane supplier if they would do something like this, and they declined because it was an "RV application". They told me they aren't qualified/certified to work on mobile applications or something like that. They had no problem making me up high pressure hoses/fittings, but wouldn't touch the actual plumbing.