The only thing you need an original Owner's Manual for is for the parts and components that Fleetwood manufactured specifically for that camper. And other than the frame and skin, that's probably few or none. As wa8yxm
noted, you can use any RV parts that will fit. And you can even use some parts from Home Depot, such as faucets and water lines. Just like fixing things in your house, you need basic plumbing and electricial skills, with a smidgeon of carpentry and decorator skills. Unlike a house, your wiring is probably for both 110 volt and 12 volt and those are different. Most things are 12 volt, such as the water pump and lights, and the furnace fan motor. 110 volt is used for the AC and for the wall plugs. Your antique trailer probably doesn't have a microwave oven, but if you add one it will require 110 volt.
Spend some time on a good RV supplies website, and you'll pick up hints of what is available and how much it cost. RV components such as refrigerators cost a lot more than they do for a house. They have to run on both 12 volt when you're on the road and 110 volt when you have full hookups, and no ordinary kitchen refrigerator has to do that. Some even run on propane for boondocking without running down the batter(ies) in the rv. So the RV miodels cost more. One big RV supplier is
For example, note the variety of RV refrigerators they have available. Tiny 1.7 cu. ft. to huge 12 cu. ft., with or without ice makers, two-way or three-way (3-way means it can run on propane, 12-volt or 110-volt), cost from about $500 to over $3,000. The specs for each of the models include the dimensions you need to see if it will fit in the hole you have.
Of course, if the one you buy doesn't fit, then use your carpentry skills to make it fit, and use your decorator skills to make the hole where it fits look good.
Trailer axles. One big item to check on your antique trailer is the wheel bearings. It's no fun replacing a burned-up axle in the middle of nowhere (been there, done that). So you want excellent preventive maintenance to keep that from happening. Remove the wheel bearings, be sure they are perfect, then repack them by hand. If the wheel bearings are not perfect, then replace them with new ones. If the parts around the bearings, such as hubs and spindles, are not perfect, then I'd replace the hubs or entire axle assembly, depending on the brand of your axles. If you have Dexter axles, replace the hubs.
Trailer Hubs and Drums | etrailer.com
For Al-Ko axles, replace the entire axle assembly, including the hubs.
Trailer Parts from trailerpart.com