As luck would have it, I am staying at the Hilton at LAX right now, so perfectly placed to answer your question.

The Hertz cars at the Hilton are all the regular sort of Corolla, Camry sort of stuff, and your chance of anything other than a marginal upgrade are low. They don't keep that many onsite, as they are simply on level -6 of the hotel car park.

However, it is about a 15 minute walk from the Hilton to the Hertz facility, so do what I did/do........... when you arrive at the airport, take the Hilton shuttle to the hotel. Then next morning walk down to Hertz, where you will have a much bigger choice of cars, and your chance of an upgrade is much better. ( I got a Merc GL450 for the price of a Corolla without asking yesterday even though they had loads of cars. Someone must have decided to treat me!)

By the way, the Hilton's own carpark is now $27 a day for hotel guests (or $35 for valet). As long as you don't want in/out privileges, park in the facility next door, for around $13 a day!