A "B" is a van converted to a vehicle for camping (like VW campmobile and Westfalia conversions) or to live in (many of today's Roadtrek, Pleasureway, Sportmobile conversions, many more manufacturers in the past).
C motorhome is a house, usually a box, put on a cab-chassis, mostly with a van cab today, but others. Your Toyota motorhomes are type C, on a small truck chassis (HiLux) no longer imported, but higher capacity than compact Toyota trucks made in the U.S. today. Only the truck is Toyota, at least five manufacturers built motorhomes on the Toyota truck, quality, including how well fit to truck capacity, varied.
Type A motorhome is built on a bare chassis, no cab. It can be bigger or smaller than a type C. Most offerings today are bigger, but earlier A motorhomes can be quite small, and small ones are coming back.
Don't assume that because a motorhome on a Toyota truck is small, that it will be easy to drive. Overloaded, underpowered vehicles make for a challenge on todays roads, where most family cars have better performance than the "performance cars" of the '60s through early '80s. The Toyota motorhomes were made for a kinder, gentler driving environment.
At the age of these Toyota motorhomes, to buy you need to be agood judge of the condition of the truck, quality and condition of the house on the back.