Bonnie,

We use the driveline disconnect that Remco towing has. For our E-350 1ton Airstream Ford van, the cost was about 1200 dollars installed. See www.remcotowing.com or call 1800-228-2841 and talk to Calvin.
There is also another small independent company in Tucson. Do a Google search using terms like "driveline disconnect."

Good Luck