During our pre-buy our salesman was very feature savvy. We opened every drawer, door and hatch and asked lots of questions most of which he had heard before I am sure.
During the PDI the technician showed us everything including how to light the oven, where the water pump, water heater and TV antenna crank were. We took pictures and had another tech show us where to put the jack to change a tire.
The dealer supplied the WD hitch as part of the TT deal. They explained the hitch setup so I would understand what it did and how it worked.
What did we miss? I took me a while to find the low point drains and since it was a used TT, several of the drawer slide mounts were broken. Apparently a common problem. I repaired them myself.
My DW and I are not afraid to ask questions and re-ask if we do not understand.
Remember, the dealer does not know how much camping stuff you can't live without, nor do they know the max cargo rating of your vehicle. They do not know if you boondock or KOA camp. They read the same truck info sheets you do.
The dealer wants to sell you the TT you want, not the one they think you should have.
We do have a number of well established dealership and we spent a long time deciding on what we wanted. We even rented a similar unit to see if we liked it on the road.