Hard to tell what brand from that photo, but there were dozens and dozens of RV manufacturers in that era that made similar units. It's just a Class-C RV, nothing atypical. They are everywhere in the states.
The chassis was supplied by GM. The body/coach would have been built by a coach manufacturer. There should be a serial number plate inside on the wall or door jamb area that says who built the coach.
Whatever the body is, it sure is a clean machine on the outside! No idea on the body manufacturer either but the chassis is definitely a 71-77 G30. 1978 received a much different grill. Most G30's had 350 small blocks but the 400 small block was an option for a few of those years. Both were backed by a TH400 transmission in the G30.