Your landyacht Motorhome has two to three places it can leak on the passenger side near the window.
The cap, as you will see when you get on top of the unit is attached to the roof and capped with a cover. The cover will vibrate and shift as you roll down the highway. You will probably want to cover all seams with a godd coat of sealant. There are many to choose from. Kool Seal is a popular brand. But an elastomeric compound will do.
Second location is the joint that runs down the lower edge of the cap, just above the door, some good silicone for that spot.
Third spot, the window behind the passenger seat. The water can leak in there but find its way to the passenger door.
again silicone this window if its leaking.
The Motorhme is fiberglass on aluminum amd you can use silicone rather than vulkem which is recommended for the aluminum trailers. (not that you could not use Vulkem)
Congratulations on your purchase,