Hi,

Would love to connect a USB charger for my phone on the bike. Yes, I can go straight to the battery, but it'll drain and I don't want to deal with remembering to turn things on or off. So I'm looking for a switch circuit that I could tap into.

Looking at the diagram, the only easy thing I could find is the horn, but not sure if that's such a great idea. Anyone can recommend another switched circuit?

 

There is no reason one couldn't use the horn circuit.
Another option would be your headlight circuit.

 

I installed a voltmeter/USB that I just unplug from teh battery tender cable when not in use. E-Z.

 

I think that one makes more sense. I was thinking if there would any concerns with the lights going dim... . Do you know if this should be done in parallel or can be done in series?

Yeah, that's always possible, but I know me and my forgetfulness, which will (100%) result in one empty battery.