The battery should be 12.? with the engine off, and 14.? with the engine on. If it’s the same with the engine on and off it’ll be the alternator (and/or connections to it), easy thing to check.
I’ve got a gadget which you plug into the fuse box, take a fuse out plug it into that hole, then try the next, etc, etc, this tells you if a circuit is drawing current when you think nothing is on. A cheap and very useful tool, but I can’t remember what it’s called. Also, on my van, I don’t know about other cars, there is a special ‘hidden’ fuse box, there’s one in the engine bay, one behind the glove box and one behind the one behind the glove box, took me ages to find that, and that’s of course where the blown fuse was, everything else was good.