If I had been in your situation and travelling I would have had my own Garmin Nuvi with me. So that probably would've helped my argument, since I'd have to be an idiot to pay for NL on top of that...

Also I agree with lardiop. I know it can be difficult to keep your cool when the agent employs the "Customer is always wrong" attitude (which is standard training at U-Haul, I speak as a former employee), but keeping your cool only makes the agent look worse to other customers (which makes them think twice and hurts business) and raises your position. If you descend to their level, most people automatically sympathize with the agent rather than you, who is clearly in the right. Not saying you should be overly concerned with what others think, just that staying cool and collected is an easy way to keep the leverage on your side.

And a final note, having been a young renter all my life (not 25 yet) I know how it can seem like the rental companies are against us. I've always maintained my cool and had professional transactions with car rental companies, and many of them have upgraded me to cars I'm not technically allowed to drive, likely because they didn't realize I was still under 25.