Ok 2 time Sprinter owner here. Your owners manual is your best reference for this info. 15k miles or 1 yr from date of first in service, (your purchase date) for your oil. It takes about 13 quarts in the V6 of a special 229.52 European spec 5w-30 or 0w-30 oil. You MUST use this oil to maintain warranty. Mobil 1 is the preferred brand, and it usually has to be ordered if you buy at auto parts stores. It's not a normal stock item. And it's not available at WalMart.
Stick with an OEM brand, (Mercedes, Mann, Hengst), of oil filter as some of the store brands are made in China and have been known to collapse, ruining that $15,000.00 diesel engine, (on your dollar).
If you have a 2015 chassis, I believe you have a 4.75 gallon DEF tank. If you are adding as much as you say, you are not getting it full. It needs to be filled up to get your proper miles and time between refills. You have roughly 500 miles to refill after the DEF light comes on. We get about 5k miles per 2 gallons. We have a 3.2 gallon tank. Idling and slow speed, stop & go use more DEF.
The engine fluid maintenance on a Sprinter is critical to it's life, and the correct fluids have to be used, which are mostly not off the shelf. Sprinters are great vehicles, but are very finicky in their requirements.
If your Sprinter has Continental tires on it, expect about 30k miles for their life, and I suggest you get a front end alignment ASAP for max life. All RV's should get an alignment around 5k miles if bought new because they were aligned at the chassis factory with no weight on them.
If you take your Sprinter to a dealer, be sure you understand what service they will do and if it's something you can do yourself, (DEF, windshield washer fluid, cabin air filter), do it yourself. MB dealers, not all of them now, are notorious for presenting an unknowing owner with $3,4, & 500.00 service bills.
Oh, and one of the big Sprinter no, no's is don't let it idle at it's 790 rpm idle speed for more than necessary as it can cause all kinds of emission related issues requiring a dealer visit. And don't be afraid to drive it hard once in awhile. Don't putter it around...
Any other questions I'll try to help you with them....