How to keep wasps/hornets away from my truck?

Obviously some wasps survive the winter or they wouldn't remain. I suspect that they have been using it for a nest and have come back to nest in the same spot. It may be that your sister is correct but I have recently painted my eves on my house and wasps came back to very nearly the same spot even after painting. I would suggest that you spray it with insecticide. There may still be a nest. There are a lot of places for the wasps to hide their nest particularly if it is a new one. Watch where they go if you can and maybe they will show you where it is. Wasps can be very determined. I would hate to have one in the car with me when I was driving. You might try a cover for a while. Good luck.