The M series chassis is different than the B series. The M series was a motorhome specific chassis that Dodge made in the early years. Starting around 1973, many motorhome manufacturers started using the stock Sportsman B300 chassis for Class C motorhomes. With the stock Sportsman, the chassis VIN will identify the engine information. However, you cannot use an online VIN decoder for newer vehicles - you have to use a manual (compare the digits in order to determine the specs) decoder
Like This (sorry the above one was for dodge trucks - this is for the vans).
My VIN starts with B36CF5 - That correctly identifies it as a B300 sportsman maxiwagon chassis with 360/2BBL engine built in 1975. The only change is I changed out the OEM carb for a 4bbl so that is now incorrect (I've noted it for any future owner).
