Welcome to the forum and congratulations on your new Ericson 28. An internet search could not find a manual on a Ericson 28 that would show an electrical schematic diagram but did find one for an Ericson 32 attached so unless the forum can help, you might contact Ericson Yacht Organization below.
Congrats on finding your Ericson. Welcome to the forum.
As you will soon discover, finding "ORIGINAL SCHEMATICS" while it seems like a needed item is most of the time of little value on a boat of 40 years. Previous Owners have likely altered most if not all of the systems.
We buyers of "Mature boats" have to make our own schematic. This usually means crawling through the boat and looking in all voids and compartments to discover what is inside.
Too bad you do not have a laptop with Microsoft Office because "Visio®" is a very easy program to use. I used professionally for conceptual chemical engineering designs. I understand SmartDraw reportedly offers a powerful and easy-to-use solution if you want a Visio® alternative on a Mac. SmartDraw reportedly offers more templates, better tools, industry-leading automation. Also take a look at the link below entitled "Best Free Drawing Software for Mac" for potential drawing apps for Mac.