EDIT: Ended up being bad battery. Replaced under warranty.

2004 F250 SD 5.4L. Put new battery in in Feb this year. I drive the truck about once every 2-3 weeks and it has bee running fine. Tried to start a few days ago and completely dead. Put the charger on overnight and still nothing. Battery read 3.9V. I turned the key to ACC and I checked all light switches and doors to make sure nothing was on - Did this with charger hooked up to get some power. I pulled / pushed the hazard switch to make sure it was off and I could hear a relay clicking and saw the exterior cab lights over the bed were ghosting on / off but hazards not flashing. Disconnected battery and charged it again and it shows 14.3V. Connected and still nothing. Battery (connected again) shows 4.2V so I think I have a bad ground or short. Strange because it drove fine to where it is parked so I'm thinking maybe a short caused by driving and a wire wearing down. Any ideas appreciated.

 

Bad battery based on the readings. Pull it out and take it somewhere to have it fully tested.

 

I'll pull it in the morning and have it checked.