I suggest that you find someone familiar with RV's to spend a bit of time with you going over what you do and don't have. Even spending $100 to have a local RV dealer mechanic go over everything won't be a bad investment. If you post where you are located there may be a list member near you that can also help out.
Now about your questions:
You Scamp is over 35 years old and hard telling what a list of previous owners may have changed/removed/updated. In many cases earlier FGRV's didn't have any kind of converter/charger and depended on the TV's (Tow Vehicle) charging line or an external battery
charger to charge the battery. Plugging in, often only gave you an outlet or two and one or two 120VAC lights
The Power chassis/Converter of choice is the Progressive Dynamics PD-4045. It will provide an AC distribution section, a 45 amp DC converter with 12 fused circuits and a 3 stage battery charger. They are available for less than $175 on-line. I have installed 7 of them, including two in 13' Scamps, without problems. I believe that they are now what is being installed in new Scamps as well.
As far as the original wiring diagram, that may be of little value because, due to the age of the trailer, many changes may have been made. I suggest, basically, starting over in that department. In a 1979 Scamp there isn't all that much to wire, usually 3 light
fixtures, sometimes a refrigerator
, and little else. The trail/running lights
are completely separate from the coach wiring.