If you plan on buying new, the stuff that typically doesn't come with it are sewer /water hoses, tire chocks, tire covers for covering while in storage, patio mat, chairs, small outside table, portable grill, maybe a set of tools, etc.
If buying used, then a lot of this stuff may come with it. Either way, you will want a good weight distributing hitch.
If you want to rent, take a look at
RV Share to find a unit to rent - but these don't always come with a WDH, so you may be limited to local campgrounds.
Do your research on payload - don't rely on tow capacity. Figure what you can tow before you start looking.
Good luck