The manual for your trailer would have been a 3-ring binder with individual manuals from the manufacturer of each component, such as water heater, refrigerator, furnace, etc. Most of those manuals can be found and downloaded online.
HR did have an Owner's Manual, but it is a small booklet with very generic information in it. I've attached a copy of a 1978 manual for you. A 1973 would have been very similar.
curmudgeon: noun; a crusty, ill-tempered, irascible, cantankerous old person . . . .
79 27' Holiday Rambler Statesman, 78 32' HR Imperial, 85 36' HR Imperial 5th Wheel