[hydrogen7800]

Ullage thrusters are used exactly as you described, for tanks that do not have PMDs. For example, the large second or third stage main propellant tanks which need to be lit in zero G. On Saturn V, the ullage motors themselves were solid rocket boosters mounted to the aft end of the stage, and thus did not themselves need PMDs, nor access to the main liquid propellants. On some videos of stage separation, you can see these solids firing from the point of view of the jettisoned stage facing forward. (In fact, I think there were also retrograde solid thrusters at the fore end of the stages to aid in separation.) I suppose you could call ullage solid motors PMDs, too.

For an excellent deep dive into the systems, operations, and functions of the Saturn V, get a copy of "How Apollo Flew to the Moon" by W. David Woods.