DEF usage is much smaller/slower than the diesel fuel itself. Do NOT put any DEF in with the diesel. If you run low you should be getting a warning , will be slowed down in limp mode and if you keep going eventually the engine could shut down to protect itself. As stated earlier, you can buy DEF at most truck stops in containers or via a pump in the truck area. Walmart sells in 2.5 and 1 gallon jugs. 2.5 gallons is just under $11.00 so yes, is cheaper from a DEF pump at a fuel stop.
Please read your manual. I am also fairly new to diesel and I had and still have lots to learn. Very different animal than gas......... Read about proper starting, cooling down and additives that may or may not be recommended for your unit. Your manual gets you the best answers for your system. Happy RVing!