I'd also think that slumlords would be very reluctant to part with money for repairs, or payment of your invoices. An exception may be if they have a corporate office you could deal with instead of the local manager. My btother-in-law has a cleaning business & the local office wasn't paying, but he gets his pay from the corporate office now & it's working well for him.
I've had a remodeling business for 10 years as a remodeler, and ten more as an electrician in residential electrical (recently some profitable commercial work). The upside of remodeling for Mom & Pop is a check when you pack up your tools. It's much easier to keep the business going when the cash flows.
Sixty to ninety days can be a killer until you get a big fat bank account.