You probably won't find a manual for it, if there ever even was such a thing in the first place. RVs usually come with separate pieces of documentation from the component manufacturers, e.g. a stove manual, a water pump manual, etc. The body manufacturer may have had a few pages of overview, but rarely anything comprehensive or even very useful. Your best bet is to identify the brand and model of the major components and then try to locate manuals for them. Some will be available online at various websites (not necessarily the component company's site) - Google will help locate them. Others you may not find, but you may find a nearly identical model that will be close enough for most purposes. and perhaps somebody here - or somebody you meet in a campground - will happen to have the same one and will copy it for you.
So far I've owned two 5ers one was an Aljo and the current one is a Cardinal. In both cases they came with a "users manual". In both cases they are just about useless.
The problem is that they do not write them for a specific unit -- they are so generic that the information is FAR from specific. Some of the useful stuff is how to sanitize the water system (this is also generic, but useful since they tell you to use chlorine - so much per so many gallons.) They also give you general info about safety and keeping your tire inflation pressures up to snuff (again very generic - basically telling you to inflate to the pressure marked on the tire etc.)
So there is some helpful info, but most is quite vague. I doubt that you will be able to fine a book for your unit, but if you do it won't provide a lot of info. Sorry
On the other hand... I just purchased a used Alpenlite slide in, the user manual is great, has schematics, routing of LP, all sorts of useful info. The Arctic Fox I had was essentially an expanded brochure.
Getting a copy of a possible existing manual for a 1978 is a real miracle.
Maybe if we knew what you were trying to determine we may be able to help. For instance, appliances will be easier to get books on from the mfg.