I would second that you should consider getting to Erie or the Lake Erie shore and picking up the Northern Tier route there. As has been already pointed out, check the PennDOT web site for the PA Bike Route system, or go further north into NY state and take NY Bike Route 17 across NY. That would put you on the Northern Tier route at Westfield, NY heading SW to Erie.
It's also possible to continue west into Ohio from PA before you pick up the Northern Tier, but the problem is that as far as I been able to find, all the bike maps in Ohio are privately published and you have to buy them in order to lay out a route through there. It's also fairly hard to find the list of sources. It would be easier and cheaper to use the PA and NY DOT sites for your research.
FWIW, I have ridden NY Route 17 from Corning to Westfield. It's a nicely thought out route with only a few hills. Can't say anything about the part of NY 17E of Corning, though. That may be more hilly since that's where the Appalachians pass throuigh NY state.