Two more thoughts.
Check the year vs the manual. I think MB is recommending either a 15k or 20k oil change if you use specified oil. I talked to the service tech at MB and he said the way we use motor homes, mostly interstates, 20k is not crazy for an oli change. He did recommend a shorter interval on the transmission 30k not 40k because of the weight. He also said that you really don't want MB to do the oil change. Do it yourself or anyplace but MB, they charge to much.
Also the sales guy may be wrong about the DEF unless you have an older one with the larger DEF tank. Mine is the small one which is filled under the hood. That one needs 2.5 gals every 2100 miles or so.
Ed and Ann in Marietta
2018 Four Winds 24 HL Sprinter Chassis
Cats = 2 (Max and Nellie) + Dog (Fiona) = Happy family.