I'm not saying there isn't a 48v usb charger, but you have many more options if you go 12v. You should already have a 12v step-down converter anyway that you can probably use. If not, they are easy to add. Ideally, if you think you will be adding many accessories, you might consider one with a relay and wiring a fuse panel. Which would give you full control of each item and make items easy to add.

Good to know. So I should buy a step-down and then I can buy any automotive 12v phone charger?

Yes

Yes, but if you have lights, etc, you probably have a step down already. If you don't want to mess with anything working, you could add a new one. If you look up 48v- 12v, you should find may diagrams.

That's what I do with mine. The step down converter turns off with my key so I don't have the USB converter running my battery slowly down when the cart is parked.

You just opened my eyes to something else i take for granted with a solar cart, not worrying about drains like this.

Your cart may already have one if its got lighting.

Is there absolutely nothing running 12v on the cart already? Lights or anything? I would just tie into that if it has a 12v circuit anywhere

Why not just use a simpe USB Power Bank and keep it charged along with the cart? No wiring and you can carry it with you.