Before you get too far in your research into MDTs, if you haven't done it already, you ought to read Larry Zeigler's article entitled, "Reasons to consider a Class 8 RV Hauler," found on the first page of this forum. In it, Larry shares his unfortunate experience with an FL70 Freightliner MDT. As Larry points out, MDTs were designed not for long distance hauling but rather, for short hops aroung town with small trailers. As such, they have medium power engines that tend to need majors at around 300k instead of the 750k to 1000k you can get out of the larger, more powerful engines in class 8 trucks. MDTs don't pull long, steep grades well at freeway speeds because again, they weren't designed to do that since they rarely need to in their normal duty. As Larry found, when pressed into service they weren't designed for, reliability can and probably will suffer. When my family first got into trucking we made the same mistake, trying to press a medium duty Intn'l into long haul service and we experienced all the same problems Larry encountered until we upgraded to a class 8 truck.
I realize that some of the members of this forum have MDTs and are very happy with them which is fine. However, I don't see any advantages to an MDT unless you are buying a brand new truck where there may be some price advantage. In the used market MDTs are actually more expensive than class 8 trucks of similar age and condition. I'm not sure why but I suspect it is because they are valuable to small businesses that need them for short haul operations. The class 8 truck will perform better in virtually every duty an RVer will put it to. The only reason I can think of to explain the popularity of MDTs among the RV crowd when upgrading from a light duty truck is the lack of a CDL requirement, since that is always mentioned in the ads, but if the class 8 truck is registered as an RV then that isn't a factor. If the MDT isn't converted to an RV, which will be more difficult and expensive if going the RV hauler route since MDTs generally don't come with sleepers, then the MDT will probably have to be registered as a commercial vehicle and the weight tax is likely to cost more than the RV registration for a converted class 8. Just my two cents concerning some factors to consider. In the end you pay your money and take your choice and everyone has to do what is best for their own situation and circumstances. Later, and good luck.