Not sure if the premier inn at St'davids has any parking.
Can you not share a room with one of your friends, I stay in premier inn's pretty frequently and often get a room with a sofa bed as well. I am sure one of your friends can request a room like this I have always found the staff to be very helpful allowing bikes in rooms etc. Rooms are charged per room not per person.
The premier inn is directly opposite the train station which will make free parking hard, if you come out of the premier inn and head left their are a few roads of small terraced houses that you can probably park in overnight but not leave the van during the day.
If you need to leave the van during the day then google this:
50.731121,-3.544311 their is a car park behind the station that is pretty good value, cross the level crossing and turn immediately left.