We offer this gasket on our site but if out-of-stock you can easily source them on Amazon or other retailers, they are standard: https://velitcamping.com/collections/air-conditioner-parts/products/velit-rooftop-ac-sealing-gasket
We recommend inspecting and replacing this gasket as neccessary to prevent leaks or water intrusion. The time period for replacement will depend on many factors. Yearly replacement is possible in some climates, but most users see 2+ years without resealing, but it just depends.
Our Velit AC units ship with gaskets to allow mounting in most applications. If you want to take it further than what we provide there are retailers who produce many different products for mounting rooftop AC units to vans:
Amazon Gasket: https://a.co/d/00EVUYUF
Sealant: We recommend Sikaflex 221 Adhesive or Dicor Self-Leveling Lap Sealant
Sealant can and should be used to help with roof rib gaps and general sealing of the gasket to the roof surface.
Lots of accessories, sealant and options available here:
https://diyvan.com/collections/ac-adapters
Common Roof Ridge Adapters (optional):
(Be sure to check with retailer that product is available and fits your specific van)
Promaster Roof Ridge Match Adapter: https://diyvan.com/products/promaster-roof-air-conditioner-adapter?variant=44083996229851
Ford Transit High & Mid Roof Air Conditioner Adapter: https://diyvan.com/collections/ac-adapters/products/ford-transit-high-mid-roof-air-conditioner-adapter?variant=44083994656987
Sprinter NCV3 and VS30 High and Low Roof Air Conditioner Adapter: https://diyvan.com/collections/ac-adapters/products/sprinter-ncv3-and-vs30-high-and-low-roof-air-conditioner-adapter?variant=44782350008539