The Shrimpers have enjoyed a good away record this season and Tilson believes his side can cause a shock at the Keepmoat Stadium.
"We wanted to get an advantage to take into the second leg, but there's still all to play for," Tilson told United's official website.
"I said before the game that as long as we were still in the tie after this match I'd be pleased. That's certainly still the case and we remain confident.
"We wanted to take a one or two-goal lead into the second leg, but Doncaster might be thinking that they've already won the game.
"We have a good away record of late and I'm still positive that we can go there and get a result."