You could also use customer types. This is a better option if there are also expenses associated with your customer types because you can filter many reports by customer type, including your P&L. Exporting your customer list is the easiest way to add them. However, this creates a separate, filtered P&L for each customer type.

You could also use classes and the P&L by Class report. The drawback to that is you’ll have to open every transaction and add a class to it. Very tedious.

Owners Equity
    Owners Contributions
        Owners Draws

Start by making sure the item you use for the principle payment on your invoices is mapped to the notes receivable account, so it’ll reduce the balance of loan. Only the interest payment is revenue.

Then either run a Quick Report from the chart of accounts, double-click on the Notes Receivable balance on your balance sheet report, or a Custom Transaction Detail report filtered for the notes receivable account. If you don’t have separate Notes Receivable accounts for each loan, and I wouldn’t, you can filter any of these reports by Name.

Make sure to filter the report to include all transaction types except invoices, otherwise it'll include unpaid invoices.