None of the car rental companies (Avis, Budget, Alamo, Dollar, Hertz, Enterprise, etc) rent towing vehicles. Their contracts specifically forbid one from towing anything behind one of their cars or trucks.
You would be better off actually checking for companies that rent trucks specifically for towing.
I checked truck rental for towing information for Los Angeles (I did a search on yellowpages.com) and found 2 companies that provide such vehicles:
Bandago -- But they only have 9-15 passenger vans (I'm sure that's bigger than what you need)
Their website states:
"We will provide a towing equipped vehicle for your enjoyment that you can tow behind the motorhome/caravan you have rented from us." --
this might be worth checking out!
Hope this helps!
Pages from this site, relating to this topic:
Can you Tow with a Rental Car?