Yes, the rental agency on Katella (quite close) is National. When we arrived from SFO/Las Vegas last month, we dropped our car there. Actually, we parked in the carpark for the PPH and checked in, then my DH drove the car to the return place (there was no charge for parking at the PPH carpark as we were staying there...) and then he walked back to the hotel (only took 10mins...
The day we left Disney, we picked up a rental car from the Alamo office there are DTD. I had prebooked the car online and didn't find it more expensive than anywhere else.
One thing to be aware of is that they have limited cars actually at that site. If you have pre-booked, they will call their other site and have your grade of car brought over. BUT, if you don't have a prebooking, it looked like they will only offer you what is on the lot.
While we were there, we saw one couple come in without a pre-booking and they wanted a small car, but they were told that the only car available was a convertible and a small car wouldn't be available until the afternoon (this was 9am). My DH and I thought about offering them our SUV with the 3 kids, and telling them we'd happily take the convertible!!!!!!!
So, my advice would be - it is definitely convenient to get the rental from Alamo at DTD, but maybe pop in the day before and let them know that you've got a reservation for the next day, that way they can have the car there ready for you - we had to wait 15mins for our SUV to arrive from somewhere else - great car though...
Regards
Jenn