Penton deemed that Simon Francis had handled the ball in the area, leaving Matt Fryatt to open the scoring with his 30th of the season, before his 31st confirmed the win and Leicester's return to the Championship.
"It was never a penalty and it changed the game," Tilson blasted. "Up until then we were really on top.
"We had a good couple of half chances and Steve (Mildenhall) has not had a save to make up until they got the penalty. The penalty deflated us a little bit."
But Tilson, whose side are now unlikely to make the play-offs, backed Leicester to succeed on their return to the second tier of English football.
"Leicester are top of the league by a long way and will cope well in the Championship," he added.