The diesel brake is where the truck uses the diesel engine to slow the truck and trailer down. It works amazingly well and makes you feel (at least I do) more in control when descending a down large hill when towing. Remember those two trailers that you are considering will weigh between 8000 lbs to 9000 lbs fully loaded and it will be pushing you down those hills and the diesel brake is a very effective weapon to help slow them down.
Also, your salesmen may be a nice guy but he doesn't seem to know anything about towing, so I would not take anything he says very seriously. Inside the drive side door or passenger door you will find a sticker on every truck that will give you actual towing and payload capacities for that truck as it is equipped. They put it on an the factory and its got all the info that you need. When I was looking at trucks I snapped a picture of sticker with my phone, so I always had that info. Lets say, you buy the 292RL and you've got it fully loaded. It would probably weigh about 8500 lbs give or take. To safely tow you would want approximately 12%-15% of the total weight of the trailer on the tongue. So your tongue weight would be approximately between 1000lbs to 1300ls. That weight comes off the trucks payload and so does everything else that you load into the truck, even the weight of the dogs and all the misc. gear. As I said above I once saw a real fancy Ram with every bell and whistle and all of that extra options brought the payload down to 1400 lbs which is basically useless for towing.
Lastly, there is one other thing about diesel vs gas trucks that you should be aware of. Diesel engines are very heavy, in some cases it can be 500 to 1000 lbs heavier than a gas engine and that extra weight reduces the payload capacity of the diesel truck. When I was buying my first tow vehicle, I had picked out a 2018 RAM 2500 with the 6.4L gas engine and its payload was 3300 lbs. I had never had a diesel and they sort of intimidated me and I had heard they were maintenance nightmares (I later learn not true). Then I did research and got on forums and they changed me mind. So I went back to the dealer and switched the truck over to one with a diesel engine. I was shocked when I saw the payload on the diesel version was reduced to 2450lbs!. The two trucks were almost exactly equipped the same. The salesmen explained that the payload reduction was caused the added weight of the diesel engine. At the time, out trailer's tongue wt was like 800 lbs so it could easily tow our trailer and I bought the truck. Unfortunately, that truck ended up being the most unreliable vehicle I ever owed, worse that my wife 1971 chevy Vega!
