Congrats and welcome to the forum. There are some video's on YouTube on Crossroads Bel Air 31.
When you say manual, are you looking for something that covers the whole trailer? Most every RV I've purchased new I get a bag of manuals. Each component, A/C, stove, furnace, water heater, etc has it's own manual.
I would suggest getting the manufacturer, model #, serial # and date of manufacture from each component and have those written down. You might be able to request manuals from the manufacturer. As for the specs on the trailer, the weights should be on the trailer tag with VIN. Sometimes there is a "duplicate" of that tag inside a cabinet somewhere.
You could look for a receipt with a name of a previous owner that may have some information. Wish I could be of more help.