Beach-Craft was a motorhome manufacturer in Elkhart, IN from 1973 to 1988 when they closed up shop. Finding a manual would be almost impossible unless someone sees your thread and can share one. Often manuals provide only general information and not specifics like where fuses or drains are located.
You can try:
Manuals - 1989 & older - RVing-The Midwest Connection
Also do a search for Beach-Craft using the Google search box at the top right side of the page and you'll find a few other Beach-Craft owners.
Best use of your time is going top-to-bottom, front-to-back in your RV and write down every brand name and model # of every component you see. Also note any fuses or circuit breakers you discover. Then sit down and do a Google search of each item with the words "owners manual" attached. Many components will have online documentation available.
You can also read manuals from other manufacturers to get an idea about proper actions setting up camp with the RV.