Earlier, Zebre had moved into a 10-3 lead, thanks to Gonzalo Garcia's try but the hosts were pegged back by a Gareth Davies effort.
They moved 13-10 ahead - courtesy of Tommaso Iannone's penalty, but Priestland dragged his side level before doing so again in dramatic circumstances at the Stadio XXV Aprile.
It means Zebre's wait for a home RaboDirect PRO12 win extends to 14 matches but Bortolami could at least take solace from the fact that it's the first time they have avoided defeat at home in the league.
"After two tough matches in the Heineken Cup we could go back and offered a good test for them, we coped well with the pressure from our opponents," he said.
"We were confident on the field and we were able to exploit their mistakes, we should be happy with the result against one of the best defences of the RaboDirect PRO12.
"There is still a little regret for the draw, but we have brought home two points and I want to see the glass half full."