Definitely new tank, not used. Check the date on the regulator to make sure it hasn't expired.
I'd recommend a new regulator. (New regulator is my preferred practice ... why deal with or work on something twice.)
Staying with a permanently mounted tank will probably be more expensive but eliminates the need to build a tank compartment.
If you choose to switch to portable tanks, understand there are significant safety concerns and legal requirements/restrictions. At a minimum, there must be a sealed 'firewall' between the compartment and motorhome interior. The door on the compartment must have at least one vent to the exterior and preferably two. If one vent, it must be located low on the door. (Propane is heavier than air and sinks to the lowest level.). Two is preferable, one located low down and the other near the top of the door.
Also, the tanks must be securely fastened in the compartment, to prevent movement.
ABYC has good standards for propane installation on boats and yachts. The added safety margins in ABYC guidance is worth applying to motorhomes. (Regulations and guidance specific to motor vehicles is hard to find and difficult to understand.)
