If you are getting it from a dealer get a thorough pdi. Videotape the pdi. Make sure they show you everything and you repeat the demo. If it opens, closes, turns on or off, moves etc you want to demo and return demo it (this includes doors, cabinets, drawers etc). Run every system including heat and AC. Make sure it warms up and cools down in the rig. Double check carefully for any signs of leaks and at that age carefully check your tires for signs of wear/cracking and for date age etc. Don't hand over money until this is done to your satisfaction. Also have them show you winterizing and dewinterizing processes.
If picking up from private seller, have them show you how to work/run everything. Again video this for a later date. The above list applies to private seller but may be limited by what facilities they have available to hook up/plug into.
Make sure you get a water pressure regulator, good sewer hose (25-50 ft worth), white fresh water hose (I carry two 25 ft hoses for those far away spickets), a splitter to plug 2 hoses into. this is the top of the iceberg. A surge protector is recommended by many but is very expensive. Look into the geo method
for maintaining your black tank and remember to always keep at least a couple gallons of water in your black tank to avoid the dreaded stone mountain from forming. Do not leave your black tank open when at full hook up, empty tank when 2/3 full (or at least add water to the tank before dumping it ) the list goes on but this will get you started.