Can't figure out why I have no power to gauges or lights, except headlight.

Did you just buy this truck?

If no bad fuses or relays, it has to be a plug or bad wire. take all plugs apart and reconnect.

run the fault codes first to see if there are any codes, next is to use the scanner to reset the ECM program
Lastly is to check the wiring circuit to ensure that you have not by-passed a control that stops the battery from going flat.
Lastly have a load test done on the battery because the gauge shows full voltage , batteries can fail under a load such as starting.

there is a regulator for the display brightness, you may have it turn'd down or it has stopped working;. it controls the brightness at which the dash lights are. usually has a speedo sign on it. goes up and down. it works with the main light and is conecter to them on the same lighting board if it has gone then a new light swtch control switch will be needed.

Symptoms point to the cluster microprocessor. How was it proven? See diagram below.

i had this happen with my dodge ram 1500. the guy i bought it from put 350 dollar moon lit lights in with leds and they looked impressive but the issue was that they didnt go bright and i could only see 40 feet in front of me and i was sure taping led flashlights would have done me more good that those stupid headlights. then i had the idea of if the brights work and the dims dont then that means no electricity is getting to the dims so i decided to bridge the power from the bright headlight to the dims and it made all four work. now they all work, and only on bright. so im figuring there is a issue with the wiring in the truck. so this is a temporary solution if you really need just plain old lights like i did because i live out in the country

take a look at the dash ground wire located next to the right of ebrake release handle. if the bole holding the ground wire is loose, which was quite common it will cause all kinds of feed back problems, like described in the comments section to your question.

Wired wrong or bad ground.

you have a short, somewhere... you need to check the brake light switch on the brake pedal, see if you have power there.

try changing the headlight switch,I had a similar problem only mine happened after I accidentaly short out when I was replacing the horn pad and for some reason when I replaced the headlight switch the problem went away.