4 wire 50 amp service is common to motorhomes. It is designed for standard residential configuration 120/240. There are typically no 240 volt loads so the system can be used with one of the many adapters on almost any 30 or 15 amp recepticle (with the adapters in place, both hots are tied together so there is 120v for all the loads and 0 across L1 and L2). The panel in the motorhome is wired like a standard subpanel with neutral and ground isolated. I'm not sure when the trend started but I have a '77 model with the 50 amp service and that manufacturer was using it in the early 70's.
Just think residential subpanel and you have a picture of most of it.
