I would ask the previous owner to make sure. However, DRV does frame in areas on the under belly and precut the outer skin on the under belly covering at each end of the slide outs where the tracks run that keep the slides aligned. It could be the previous owner either chose to tape these areas as a preventative measure like I did to assure no leaks would occur or maybe the slide mechanism needed maintenance and then theses areas were resealed with tape. if it is sealed some place other than these areas then there may have been some other issue or modification after purchase. I would certainly ask in any case just to know the history, but if no major issue, would not worry too much, just means work has been done.

This is at a dealership, so they do not know.

Do any of the other MS owners here have units with slits in the underbelly?

No slits in ours, but we've never had a need to do any maintenance on anything in the underbelly.  I would venture that if the slits you are seeing are nice and even, it is because of some maintenance being done.  Some people do a better job of "resealing" the underbelly cover than others.  One of the guys on the SOITC forums described a method of cutting an opening and then using a framework of wood when going back in to reseal the underbelly.  His description included only cutting on three sides for an access opening and that the "uncut" portion should be towards the front of the trailer.

When going back on with sealing the underbelly, the "flap" was then easier to seal with the wood framework and with the uncut part of the opening to the front, it is less likely for the winds under the coach to loosen and tear up the maintenance opening cover.

Terry