OP, that is marketing hype. Yes, a 2019 1500 in the PERFECT configuration can tow "UP TO" that much weight as long as it is perfectly set up. That much weight, PERFECTLY spread out on a flatbed trailer, PERFECTLY balanced to get you a low tongue weight without an unbalanced load, PERFECTLY set up with a single cab 2wd truck with the best gears for towing in ideal situations can pull that weight.
However a travel trailer needs 12-15% tongue weight to prevent sway. To tow a 12750 lbs travel trailer at 15% tongue weight puts your hitch weight at 1912 lbs. I'm absolutely sure that your payload is significantly lower than 1912 lbs. You want to tow a 5th wheel, which requires 20-25% pin weight. Even 20% pin weight is 2550 lbs... that is absolutely 3/4 ton territory, maybe even 1 ton territory. Then read your owners manual about tow limitations in reference to frontal area
Take a picture of the yellow sticker by your front drivers door. That lists your payload. It says something like cargo and occupants can not weigh more than 1500 lbs, or something similar to that. That will give you a much more realistic idea about what is safe to pull.
Looking at the Ram tow guide, it appears you have to have a crew cab to get the 5.7 bed. ABSOLUTE BEST CASE SCENARIO you have 1630 lbs of payload with a V6, 1530 with a V8 and 1320 with the Ecodiesel. That is before you add options that weigh more. Before you add passengers. Before you add that cooler of beer. Before you add that 200 lbs sliding hitch.
It cannot be done. You will struggle to get moving. You will struggle to stop. You will get pushed over the road. Your tires will be at, if not over, max weight. Your axle will be over max weight. Your transmission will overheat. Your coolant will overheat. Everything about the ride will be terrible, stressful and uncomfortable. You will regret having spent the money for a trailer that you are not comfortable towing. It will be an expensive mistake.
Good luck, I really hope you take advice and make an intelligent decision. At the end of the day, you are responsible for your decisions on the road. The RV dealer that sold you too much trailer isn't responsible. The guy that installed your 5th wheel hitch in too light a truck isn't responsible. YOU are responsible.
https://www.ramtrucks.com/assets/towing_guide/pdf/2017_ram_1500_towing_charts.pdf