Brian (the OP),
When my E450 5R110 transmission failed, the transmission shop technician rode right along with me in the motorhome as I demonstrated what it was doing. He also had a portable diagnostic machine with him plugged into the underdash plug while I drove. He could see the various error codes pop up in real time as we went down the road with the rough transmission jerking and jolting us as we went.
Luckily this happened close to home just before a 10,000 mile RV trip across the U.S.. I wouldn't have wanted to go through this a long way from home.