So I finished the Dawnguard main questline, went into Harkon's lair, killed all the vampires there, and every one in the Dawnguard survived. However after I killed Harkon and after I finished that dialogue with Isran thanking Serana, I can't find Isran, I've searched all over Fort Dawnguard and he isn't there and everybody in the Dawnguard survived.

So what happened? After I exited Harkon's castle I saw Isran but it wouldn't let me speak to him, I tried pushing the 'A' button to talk to him but nothing, he wouldn't even say anything and when I fast traveled to Fort Dawnguard with Serana I couldn't find Isran.

So what happened to Isran?

Supposedly hes still walking back to the Fort..like literally walking.

Just wait a couple days and hang by his room.

Yeah I waited more than a couple days, I'll check again.