Very important for both new and used units. You don't say which in your case. If used it may be helpful to have an independent RV inspection. In all cases, go over the unit front to back, top to bottom and check everything. Check to make sure everything works, open all drawers and make sure they work properly, have the salesman/tech show you how to operate everything and that it actually does. Keep a detail list of things you think should be fixed/adjusted/replaced. The list should be shorter than for a new unit, but not necessarily.
Good Luck