You will need to measure the height of your hitch and the height of your boat coupler to determine the proper drop to select. You should buy a class IV ball mount with a 2" ball. You could go class III; but that is close to maxing out on weight for you. Insofar as height (or receiver drop), that depends on your measurements. For example, I was using a 3 1/4 drop ball mount with my boat initially due to the measurements I got. However, after towing a few times it felt like there was too much tongue weight (you can feel it in the handling and braking); so I purchased a 2" drop. The 2" drop works well for my boat. Guys with the air suspension can get away with 3" to 4" drops; but I found that my Laramie (with no air suspension) sags a bit with the weight on it. Going to a 2" got me level. Since you are fairly new to this, I recommend that you purchase what they call a "starter kit"; it will come with the mount, the ball, and the pin. I also recommend getting a lock pin for your ball mount. Something like this is what you will need:
https://www.amazon.com/CURT-45534-T...s+iv+towing+starter+kit&qid=1603402662&sr=8-1.
I do agree with your RAM dealer though. If you are buying the boat new, then the dealer that you are buying the boat from should be able to set you up. What you are using to tow is more a function of what you are towing rather than the truck. Although some boat dealers don't carry all the equipment you may need. Here is another tip: don't let the boat dealer tell you that you don't need transom straps because the weight of the boat will keep it on the trailer. Always use transom straps.
It sounds like you may be new to the whole boat towing thing. There is a little more to it than just pulling the trailer. You need to know how to make sure that the boat is safe and ready for towing (front and back strapped, motor up, transom saver for motor, etc..). If you are unsure, watch some videos online or go to a local ramp and ask some boaters questions. Don't be embarrassed to ask people who have been doing it; it's more embarrassing to drop a boat or drag your motor and ruin it. Most people are more than willing to give pointers and help.
One more thing. I HIGHLY recommend getting these if your trailer doesn't already come with them:
https://www.amazon.com/CE-Smith-Trailer-Guide-Lighted/dp/B001RE2ELQ?th=1. These lighted trailer guides make life a lot easier to see the trailer position when the boat is not on it. They are also very helpful when driving the boat onto the trailer to position the boat....especially when its windy and choppy.