If the crack runs in a vertical direction towards the roof and it'a at the end (front or back) of the slide opening, it's a stress crack from the weight of the slide causing flexing or the RV frame flexing. The slide usually has something like a table and chairs or a dinette and a couch and maybe some overhead storage cabinets and/or under the dinette or the couch. That is or can be a lot of weight on the slide opening vertical members. The slide opening is simply a hole cut in the side of the RV and a header spanning the top of the hole to support the roof. Especially when bouncing down the road loaded, it will flex. I've seen several RV's with varying sized cracks present in the filon especially at the top of the rear corner. Sometimes the skin delaminates and you can see a wallow in the area and sometimes the skin cracks. Some are hairline and some are even 1/8"+ wide.
What it really tells you is that something in the RV structure is flexing and/or fatigued. Yes, it should be fixed as a crack in outer skin IS a leak thru to what's under the skin! Does that mean you should not buy the RV? Not necessarily so but I sure would check it out thoroughly for and in the full height of the wall for rot or soft spots 2 feet on either side of the crack both inside and out. Also a complete inspection of the frame under the RV and the suspension components for cracks or bending etc. No problems found, I'd probably by the RV for the right price and fix it have it fixed immediately.
It won't be fixed free so your purchase/offer cost certainly should take that into consideration. That's up to you whether to buy or not as the crack can be filled/fixed and a decent looking patch a couple inches wider than the crack secured over the crack so it does not leak. Roughed up filon and using something like marine 52100 epoxy will hold permanently. Filling the crack alone doesn't fix the leaking as the RV is still going to flex. If the crack seems to stop before going under a molding, you need to drill a perhaps 1/4" hole and the end of the stopped crack which will help the stop the crack from going further. I have not seen the crack you saw or where it runs so I'm just relying what I've seen myself.
