If I were a new TT buyer, or thinking about moving to a substantially longer TT or from a TT to a 5Ver; I would take the offer of a test drive in a heartbeat.
No, it is not going to give you the same feel of the loaded trailer but it is going to tell you about corners, highway travel, mirror visibility, blind spots and when you return it you can practice a bit of parking with the DW.
Renting a TT for a 10 day trip told me more about TTs before I purchased than all the book reading and forum information I could ever acquire.
I must say I never thought of a test drive but I sure would not have pass one up.