I would second taking I81 to I64 which will combine with I295 outside richmond and split back off. There is one big grade on I64 before you cross US15, but it is not that bad.
Another option is you could take US15 from PA down through VA (it combines with US29) till I64 and head west. It is not an all highway route and there are lights involved, but it reconnects with I64 east of the large grade. I64 is a spring/summer route to VA beach, so expect heavy traffic on a friday afternoon eastbound and then westbound on a sunday. Otherwise, the path is not that bad to take.
Another alternative you could use and avoid 90% of the DC area traffic is I81 to I66 to US17, it will drop you on I95 in Fredricksburg and avoid most of the DC traffic, from there you connect with I295 to I64 east of Richmond.