Yes, also rental cars and camper vans need a valid Bulgarian motorway vignette. Usually Bulgarian rentals do have a valid vignette arranged by the rental company, foreign renting companies most likely do not.
In order to avoid a heavy fine we suggest to make sure with the rental company upfront that the car has a valid e-vignette.
Vehicles over 3.5 tons must use the distance based toll system (BGTOLL)
Related articles:
What vehicles need a Bulgarian motorway vignette?
What is the BGTOLL system (distance based toll)?