I'd start with the manufacturer, see if you can find more info on HMS. Probably an abbreviation for something.
I'm a bit confused, though, it cannot be 40 feet long by 11 feet wide as the US in general restricts them to under 400 square feet. I think Canadian park models can be larger, so given the lack of information perhaps that is Canadian? When I was researching park models I did find some manufacturers that would build them 12 feet or even 15 feet wide. Not sure how they transported them!
To my knowledge park models that are that wide typically do not have built in tanks. It's the "destination trailers" that are 8'6" with slides that tend to have those. I could be wrong though.
What are you looking for? Weight specs or what tires to put on it? Type of frame or construction specs?
I am not familiar with park models of that era, but they are usually built almost like a house. Our park model even has a 30 gallon residential type water heater, even though the furnace is the typical RV type. All the appliances are also residential type. The plumbing is PEX as in an RV though.
I'd think you could still find manuals for the appliances online.