The Truck Camper had a chronic window frame leak

Yes, but you still have a failed seal and water will still get in.Going down the road, your camper gets pressurized and will actually pull water into the easiest path, and a failed window seal is exactly that. So while your gutter idea works for stationary use, going down the road will still show it leaking.

Like it was said, pull the window and use the tape. It's not hard to do and it's the only proper way to fix the leak.