The very biggest obstacle to resealing a shower wall is making absolutely every trace of contamination such as soap residue is removed, so great care must be taken to ensure the surfaces are spotlessly clean.
Because a van wall flexes the original sealant would have been a flexible mastic, not silicone.
Use either Soudall Fixall or Sikaflex 512 but make sure you only apply it , and tool it to a smooth finish where you want it, as once set its there for life !!!!