Your location/rules problems are what they are, as you say. We feel for you, but can't help with them.
No one here is going to advise you to do this job incorrectly just to accommodate your situation. You are getting good advice on the right way to do the job, which is pretty much the way RV manufacturers have been doing it since before you were born. That includes black iron pipe, mounted EXTERNALLY, from the tank location to the points of use, with penetrations through the floor at each point. Some manufacturers use copper from the T through the floor. My trailer has black iron nipples through the floor, with adapters to soft copper on the inside.
Whether you like copper is not the issue. It is the right material for the job, so if you want to do it right you use it.
The good news in this, since you are "pressed for time", is that you can do the propane plumbing after the floor is installed. So worry instead about doing the running/taillight wiring, which SHOULD be protected within the structure--and which is relatively easy to learn and do.