The “Ad Blue” side of things are completely only related to the base vehicle . Ie. Fiat/ Mercedes / Ford . Ther should be a section in the vehicles Hand book / Manual . This is not a Dethleffs issue , they do not move the fuel tank location.
The Ad Blue chemical is not manufacturer brand specific and widely available from garage forecourts and Motor factors , and on line if you don’t fancy carrying 5 or 10 litres home.
There are no gauges re mixture level , just remember where your fuel filler it is as more then likely along side it is a smaller filler marked “Ad Blue” and just fill it up. Do not be surprised if it does not take the complet refil contents, but save them any way.