Mobile locations. You can enter it and travel with it.
I think implementing it as a location is sensible. It has its own inventory and is a place where a limited number of people can be.
It may be locked like a building.
You should also be able to move a vehicle into buildings...
Figure out how to handle pulling vehicles. Like how to put a horse in front of a cart.
Somehow you bind the horse to it, the horse comes along on the ride and has an effect on speed.