Unfortunately, I can't help with your manual search, but I do want to welcome you to The RV Forum! Feel free to ask any questions that may come up as you use and enjoy your new-to-you motorhome!
Most RV owners manuals aren't much, mostly they're a collection of the individual manuals for the major appliances such as the stove, refrigerator, furnace, etc. You can find these online by doing a Google search for the appliance's make and model.
Rexhall went out of business a few years ago, they sold their Lancaster, CA manufacturing plant to Chinese manufacturer BYD. Former Rexhall owner Bill Rex stayed on as plant manager and BYD now makes electric transit buses there.
Rexhall continued operating for another year or so, making a few custom ordered RVs in their former service facility a few miles away. But the last time I drove by it had been taken over by a 5th wheel toy hauler dealer.
There are two Rexhall groups that I know of. A Yahoo group and a private group. Can't recall the name of that one but it's likely a member of the Yahoo group can help. Another RV Forum, IRV2 also has a Rexhall owners group.
Agree with lou, not many owners manuals provide much information. Product info for appliances, jacks, mirrors, etc., are provided by the manufacture and not covered by the owners manual.
Both this and the IRV2 Rexhall forums are great sources. The only real advantage of the Rexhall forum is the number of owners. More owners that will likely be able to answer specific Rexhall questions. However, as mentioned previously, many of the systems are generic and not specific to any particular motorhome.