I have heard that they will keep building the same E-350 and E-450 for the next 4 years. By 2018, there might be a replacement T-450 and T-550. Probably not before they come out with a slightly larger engine, such as a Ecoboost V8.
Ford has copyrights on the T-150, T-250, T-350, T-450 and T-550.
Right now, (well once Ford has caught up on T-150 - T-350 demand for the brand new model) Ford will be producing a T-350 cab and chassis. I would expect it to have either SRW or the current DRW with the 16" rims and light duty tires, so a GVWR similar to the 15 passenger T-350 DRW van - 10,500 GVWR.
So you might start to see some class B+ van based motorhomes with the gas 3.7L V6 based on the 10,000 GVWR DRW T-350. Some people will also install a service body truck - with a cab that has a factory back wall, or with a walk-through cab (such as a motorhome). They will offer both cab styles in the cab and chassis. To bad they are not putting the Ecoboost in the T-350 for now. I think that most motorhome makers will be waiting on the more powerful V6.
If you look up the Ford Towing Guide, and look on page 21, you can find the Transit GCVWR - it is limited to only 12,000 with the 3.7L gas V6 or 13,500 with the inline 5 Diesel. They do not list the Ecoboost with a Transit cab and chassis. Why I have no idea why they will not put the best engine in the cab and chassis.
2015 tow guideGood luck!
Fred.