Hey there, bud
I had a similar problem only difference is that I couldn't find John Marston near the cow house where he was supposed to be but I heard his voice. I tried to restart the game too and blah blah but it didn't work. So instead of getting mad and check up the whole internet I just booted the Red Dead Online. I wanted to try it for a while anyway. So I created my character and played for a little. I didn't even get a chance to play multiplayer yet. Only first initiation mission with horses (not gonna spoil any more).
When I came back to campaign I went to Valentine to get a hair cut and then straight to John. He was there waiting for me.
Also if that doesn't help, maybe you should load previous saves?