With an airplane you have required maintenance and each action is documented. The value of the plane is based on the accuracy of the logs that go with it. Both partners are aware of the history as well as the ongoing upcoming expenses, which are usually detailed out in hourly costs. In other words, if you use the plane for 10 hours of flight time, you may have to kick in $1000 towards maintenance costs, in addition to your own fuel costs.
All of this may be difficult to determine with a motor home but I am sure that it is doable with the right group of people. Rental companies do it by charging by the mile for use. It may be something similar to an airplane where you pay your up front costs to buy the coach and then pay a mileage cost to cover the necessary maintenance.