I've got a knee problem and sitting with my knee bent in normal position is painful; I'm more comfortable with knee straight or closer to straight.

I'd like to get from SFO or OAK to PVD, doing the longest leg from coast to coast in a lie-flat seat of some sort.

United has lie-flats from SFO to JFK, but there are no flights from JFK to PVD. The fall-back plan is to drive from JFK to Rhode Island, though that's a 3+-hour drive. I'm wondering if there is a better way—SFO to EWR, or SFO to IAD, or SFO to CLT, in lay-flat, and then a short flight to PVD.

I appreciate any input! Thanks.