As permanent residents you cannot own American plated vehicles. You can do most of the work via internet using a custom's broker. You can send requests to many and look for the best estimate.
When I imported our travel trailer, it cost about 22% of the trailer price for importacion taxes, pedimento, etc. Not a difficult task but if you go to the border with out prearrangement, you waste a lot of time.
All customs brokers have warehouses or lots on the U.S. side where you can keep your vehicle while they are doing the paperwork. They will then walk you across the border with the paperwork in hand and get the rv through without a problem.
I've done it twice with different brokers and the procedure was the same.