Been thinking about this topic and I really can't come up with a time period. To me Golden Age implies a peak but every couple of years the chassis and house builders come up with something new and innovative the improves the overall experience. Heavier chassis, slides, all electric rigs, front overhead beds, lower cost diesel power trains. The RV industry took a pretty hard hit 10 years ago and those builders who weather the drop in sales more or less froze their designs. But with the recent improvement in sales I think we'll start to see builders producing new features with every model year. Maybe Newmar will dust off the mid engine diesel and roll out a 40' toy hauler again or the Country Coach design with the fold out deck.
We all look back with nostalgia concerning past designs but beyond the quality control issues a new coach can have are the older rigs better when going down the road or once you arrive at a destination concerning comfort or conveniences? The GMC class A from the mid 1970's was pretty cool for it's time but I wouldn't trade what I have now.