The P30L is a Solar30. I have a Solar30. The one voltage you set is the charging voltage which is also the floating voltage. That is, the controller lets the batteries get to that voltage and then holds that voltage there the rest of the day until the light fades and it can't maintain that voltage anymore.
While camping I set mine to 14.8 volts. For not camping (storage float battery maintaining only) I would set it at 13.4 or whatever depending on ambient temperatures at the time--80F is 13.2v, 50F is 13.8v eg.
EDIT: first time out, check the actual battery voltage when the controller gets to the voltage where it stops rising. My Solar30 overshoots a little on the setting (so does Niner's) so to get it to stop at 14.8 I have to set 14.3. Once you know the amount that's it, it stays the same for any setting. I can get mine to to go to 15.5 when the max setting is 15, eg.
I suppose you could run loads like that but you can also just use the 12v socket in the Winegard TV panel. If you have an inverter, you can use that with whatever charging adapter your gizmo uses. Some inverters have USB for that. When on whole house inverter you can stick one of those 120v-to USB adapters in any 120v receptacle in the rig and use that.