Depending on the hour you are going to hit I270 the US15 route is not bad. There are some lights in Dillsburg, but its 65mph limit south of Gettysburg and 50/55 though MD to the I270 exit, but taking I270 you avoid the 2 lane section of US15 which can be congested during the day. However, depending on the time of the day, I270 towards DC, or I95S from basically the beltway to US17 can be a traffic nightmare.
KristinU's route is great, its mostly highway, there is a flying J as you cross into the VA line where fuel is cheaper then PA, MD or WV with RV islands. There are few lights on 17 here and there but mostly clear sailing.
And your last question, having propane is not an issue. There are only a few restriction areas and those are areas you are trying to avoid anyway for traffic reasons. In MD you just cannot use the tunnels with propane meaning you have to go around the baltimore beltway. But from CT to MD the tolls are worth alternate routes.