Goals from Jason Demetriou and Sam Mantom had put the Shrimpers ahead after Rob Kiernan's own goal had given the Millers a half-time lead.
But Ladapo's effort on the hour mark meant Southend are still waiting for their first home league win of the campaign.
Campbell, who took charge in October, has seen his side take two points from their last three league games but they sit 10 points adrift of safety.
Rotherham broke the deadlock in the 30th minute following a goalmouth scramble from a poorly hit Jake Hastie corner which eventually resulted in Kiernan putting the ball into his own net.
However, the Shrimpers fought back after the break and equalised from the penalty spot in the 49th minute, with Demetriou on target after Chiedozie Ogbene had been penalised for handball
Just three minutes later, Southend went in front with Mantom sweeping home a low, left-footed shot from 14 yards.
But Rotherham hit back on the hour with former Shrimpers striker Ladapo firing home from close range to ensure the game ended all square.