08 Tundra No Start electrical problems

Hi I have a white 08 Tundra SR5 I just bought a couple weeks. I went on vacation and when I got back home the truck was dead. I jump started it and it was not charging so I changed the Atlernator and put a new battery. It's started right up. Next day I am bout to head to work and it's dead again.

After work I started checking fuses and all the fuses are good. I started to check power and I get 12v at most of the fuses at some fuse I get 7.5v.

I started checking for voltage at the start relay and I get 12v on the coil side and the switch side of the relay but when I checked for the ground I get 7.5v. I am not sure what that means. Is there a short??? None of the fuses are blown. I also noticed that there is 7.5v on the chassie also.

I charged up the battery still got nothing. The truck won't even take a jump. Sometime the no power on the dash nothing come on and sometimes I do get power on the dash but as soon as I turn the key to the start position I lose all power.

Has anyone ever experienced this or something similar? And advice would be greatly appreciated thank you in advance.

I would really like to drive my truck I just bought it and only drove it a handful of time.