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The DEF tank in your 2018 Dodge Ram 3500 can hold up to 5.5 gallons, enough for around 7000 miles. You can top it up with AdBlue, a commonly found solution at truck service stations. Even though urine and DEF both contain urea, you can't simply pee into the tank because DEF has a much higher concentration of urea at 32.5%. Your Dodge's sensors can detect any improper fillings. When the warning light goes on indicating low fluid levels, take it seriously - your vehicle won't restart if it runs out completely. You'll need to fill the tank completely to get your truck moving again.
Just like how gas mileage can vary, the amount of Diesel Exhaust Fluid (DEF) used will depend on the weight you're carrying and your driving habits. For example, when towing a 19,000 lb. 5th wheel RV with three axles at 65 mph, I get 9 miles per gallon and use about 1 gallon of DEF every 500 miles. This includes driving on both flat terrain and mountains. However, if I increase my speed to 75 mph, my fuel efficiency drops to 7 mpg and my DEF consumption increases. It's not realistic to expect a tank of DEF to last for 7,000 miles unless you're driving empty, which is not how these trucks are designed to be used - that's what a smaller 1500 truck would be for.
The def tank of the 2018 Ram 3500 has a capacity of 8 gallons for fuel.