We have a 16 gallon DEF tank along with a 230 gallon diesel tank. I tend to fill the DEF at every 4th fuel stop. The chassis manufacturer moved the DEF filler to the driver's side a couple of years ago. The fuel and DEF fillers are behind the same door about halfway down the side of the coach.
We have just over 12,000 miles on the coach now and I recall going about 2,500 miles on the DEF tank. I'll check my receipts when I get back to the coach (in storage).
MM.