That trailer is 34 years old, so you're probably not going to find out much about it. But it was an expensive, well-made 5er when new, so it's probably worth rescuing.
An Owner's Guide or manual probably won't have any info in it except for the frame of the trailer. The original Owner's Guide included separate brochures for the various components of the RV. Things like the AC, furnace, water heater, water pump, range/oven, electric power distribution, ect. If any of those components goes on the blink, then get the manufacturer's name and model number off the component, and contact the manufacturer of the component.
A Suburban water heater in a Holiday Rambler is going to be identical to a Surburban water heater with the same specs in any other brand of 5er. Since you have a gutted 5er, you probably need to install all new appliances anyway, so just buy the right type and specs of appliances. If you had a 6-gallon water heater originally, then any new 6 gallon water heater with the same specs of propane/12-volt/120-volt will probably fit in the hole. In a 30-year old trailer, it was probably propane fired but with 12-volt controls.
Holiday Rambler went bust recently, and were acquired by Navistar (owner of International Truck and Engine). So they are up and running now for 2014 models, but I don't know how much info they still have for the 30-year-old trailers. Maybe contact the new Holiday Rambler division of Navistar and ask?
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