Welcome to iRV2.
1980 , I thought that was the first year of the 17 digit serial numbers ; I will have to check that out .
Also 440 in 1980
, I'd ordered a 440 in my 1979 truck and couldn't get one , I was informed there were no longer in production, biggest engine was the 360. .
Midas , would be the Coach builder ; Sportsman was Dodge's label indicating the high end interior trim package.
Chrysler corp . was in DEEP financial trouble back in the late 70s early 80s and had massively overproduced Class A motor home chassis , and was fined heavily for re-serial numbering one and two year old chassis and selling them as current year production . I hadn't heard of that being done on the cutaway chassis , but if you do have a 440 , you might have a late 78 early 79 chassis and the chassis wasn't delivered to Midas and built on till 80.
The " D " designation was for two wheel drive pick up trucks .
Your chassis would be B 300 and there can be some differences in the front end parts relating to axle weight rating.
Did you find any tags indicating ; front and rear axle weight ratings , as well as the GVWR?
I'll do some searching for info on the old serial numbers and post my findings .