Things did change (a lot) from 1968
to 1972. The trailers were 6" narrower in 1968
(they started being wider in 1969), and all the coach lengths grew 1 foot starting in 1969 as well. By 1972 a lot of the interior stuff was completely different (plastic and veneer instead of real wood), and the interior and exterior lights were different as well. Trailer ball size increased from 2" in 1968
to 2 5/16" in 1969, and you will hear a lot about things like "tambour" starting in 1972, that you don't have to worry about with your 1968.
1968 was the last year of the infamous Corning Unobtainium windows.
All that said, many things stayed the same, such as the brakes, electrical, and water systems. 12 volts DC is still 12 volts DC. Propane systems are basically the same, but I don't know if there is enough stuff in a 1972 manual you can use to consider purchasing one.
Good luck with your new purchase, and remember, getting htere is half the fun!