Soft copper tubing, as has been suggested, is commonly used.
In most applications, it works pretty well.
If you plan to do much off road camping, such as gravel roads, etc, steel pipe may be more suitable?
Soft copper can be run through conduit as well.
Some folks run it through the frame tubing, and others use an old piece of garden hose to protect it.
Since the tear will flex somewhat, provisions should be made for the gas line to be able to flex slightly.
In other words don't make the entire system rigid.
Most folks will use a flexible rubber hose (high pressure) to connect the tank to the regulator.
Then they go to copper or black pipe for the run to the rear of the trailer.
Some folks run copper right to the tank, with the regulator wherever it will fit.
Soft copper is flexible, but much care must be used not to damage it when changing the tank...not the best way...
Once at the rear, the copper or steel should terminate at a tee, or other fiiting under the trailer.
From that fitting there should be a continious run of line to the stove, or other appliance.
The idea is to have all connections/potential leaks outside the trailer.
Obviously the final connection at the stove, or appliance will be inside, but the risk has been minimized as much as possible.
With the flexibility issue in mind, the last leg of tubing, the one that goes from under the rear to the stove/appliance inside is best run in flexible rubber hose.
I prefer high pressure hose not because of pressure, but because it is very strong and damage resistant.
Another important reason is that most people will move/lift the stove to clean under/behind it, having a flex hose there will minimize the potential for the final connection to become loose from movement.
Obviously those who have pull out stoves will already have a flex line, and those whose stoves are permanantly fitted into the counter won't need one.
But any stove not permanantly mounted should have one.