Campbell was on target in the 64th minute and the Leeds boss felt his side were denied a certain penalty after visiting skipper Mark Hudson appeared to handle the ball.
Warnock said: "It is a cruel game sometimes. Everything about today was cruel.
"We missed open goals and the young lad (Sam Byram) made a schoolboy error for their goal.
"I thought referee Mike Dean was brilliant but he didn't see the handball, when we all appealed, because he was in a bad position and should have been in a better one.
"He had three or four bodies in the way. The linesman could have helped him out but they don't, do they. Then there was another handball in the 93rd minute and that wasn't given either.
"Our two centre-halves were magnificent. We played some good stuff against the top-of-the-table team and we were better than them, so it's very frustrating that we got nothing."