Without seeing the gap your talking about, I am thinking what your looking at is the space between the tub upper rim and the side panel. RVs are different then homes in many ways, that gap is needed and normal unlike a stick built home there would be caulking or grout making a water proof seal. RVs have a lip up behind the panel.
Advice would be to take a friend over to look at it who is a RVer. On this forum there is a PDI printout to help a buyer check off everything as you look the unit.
Telling you to look/check out the roof for leaks or brakes/tires etc. Would you know what good and bad looks like? It is nearly new and likely not having issues but, best to experienced eyes check it out. To go through the whole unit, testing everything and you should, may take 3-4 hours.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed 3.55 rear end lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.