Nissan versa flat towing

The dealer is being a bit cagey about the warranty. Things you do to a car don't void the warranty, though if you damage something by doing stuff outside what the manufacturer approves/recommends (or, for that matter, if third-party parts/upgrades cause damage), coverage for that damage may be properly denied. It's theoretically up to the manufacturer to show that the problem was caused by what you did/installed rather than by a defect in their materials or manufacturing. In the case of flat towing, I suppose it would be pretty easy for them to say with appropriate authority that a transmission failure was caused by that if they don't permit it per the owner's manual.

The net effect may be similar, in that you're on the hook to fix the problem on your own dime, but the warranty is not technically voided in any legal sense of the word.