First of all, welcome to the Sunline Owners Club. Many members are likely camping this weekend so it may be a couple of days before others respond.
Unfortunately there's no such thing as a camper that doesn't leak without scrupulous maintenance. They certainly shouldn't leak new from the factory--and I'm glad to hear the dealer fixed that--but, because of early model turnover yours could be 5 years old already. The Dicor lap sealant that seals the roof edges and goes around all openings, like the vents, is put on in big gobs because it is meant to flow around and really seal things up. But, like any other caulking, it is not permanent and when it comes off it can come off in big gobs too. Towing down the highway is like hours of 60 mph wind buffeting a house--they'll leak too.
I'm reading a little in between the lines here that maybe you don't do some of the maintenance yourself. You really can't pay people to inspect your roof, you have to do it yourself. I have the best 8' stepladder money can buy--I'm a big guy and this is not the place to skimp. I can't remember how the grades work--it's grade 1 or professional grade I believe. My roof is about 9' off the ground, your SR roof might be a bit more, but an 8' stepladder is still plenty of height to give a safe comfortable view of the roof.
Even on a brand new camper, the roof seams should be inspected maybe half a dozen times a year--definitely in fall before storage and in spring during cleanup and certainly after any longer road trip. It sounds like you wash the trailer regularly. Add a roof inspection to your routine. It takes 5-10 min more to get out the ladder and inspect the roof. Start at one of the front corners and move the ladder all the way around, maybe 5' at a time. Climb high enough that you've got a good face on view of each section of seam at the roof edge and check that it is intact, not peeling or brittle. Pay special attention to the corners. Climb one step higher and get a good look at the seams around the roof vents and stacks etc. It is not necessary to get on the roof to do the inspection and you'll soon learn what good sealant looks like. Don't get on the roof--it's not walk-on, but come back when you need to know how to support your weight if you do need to get on.
If my experience is at all typical, the front corners is where the Dicor lap sealant is most likely to break away. I've never had a leak, but I've also redone one of the corners twice. The first time the camper was less than 2 yr. old. Look for the leading edge of the bead to be loose or even lifted off. In your case it may even have broken away. Dicor flows out in a smooth bead. If there is any kind of rough or irregular edge to the bead, it likely has broken off and you might even be able to see the source of the leak. In any case it doesn't take much for water to find a leak, especially driving down the hwy. If you need to redo any part of any bead, break away any loose pieces with your fingers. Do not use tools or try to peel off the bead to start over--Dicor lap sealant is not at all like household caulk--it simply flows overtop of everything.
This is the product for the roof:
Don't get this one--it's not free flowing:
You can buy Dicor at any RV dealer. Don't be afraid of using this stuff. It does not go on neatly. The big gobs you see around the roof vents is not sloppy workmanship--that's just the way this stuff is. You just squeeze it on and let it flow. Don't use your finger or any tool to try and smooth it in any way. Nobody is going to see the roof except you. Just don't use so much that it runs down the siding.
Here's a link to the latest thread on roof leaks:
I don't know about replacing the paneling inside. Hopefully somebody else will chime in with that. I believe most rvs use the same stuff so you may be able to order some from any dealer.
I know you said the leak appears at the window, but it's hard to tell where water finds an entry. Front windows are leak prone also. Post a photo and someone can be more specific in their reply.