Off side (LH) propane cylinder has that single stage regulator to drop cylinder pressure down to 30# PRIOR to going across front of rig to the 2 stage main regulator where system pressure is dropped to 0.4#
Reason for that 'regulation/requirement' is full cylinder pressure (between 100# and 200#) in that long hose over time can become liquid when not used.
Liquid will destroy a regulator
The other NFPA Regulation for propane is 5.3.11.1
**The regulated high pressure piping shall be located entirely on the exterior of the vehicle or in a compartment vapor resistant to the vehicle interior
So short high pressure hose to 30# regulator is inside the off side propane compartment then Low pressure propane runs across front of RV INSIDE the front cap.