Dropped my 2400R off at the local MB dealership today to deal with a climate control issue and asked about the spongy brake issue. I don't experience this issue but I have read a lot of the related threads. The dealer in Grand Blanc, MI said they hear this complaint a lot on the Sprinter chassis that have been modified by an upfitter (be it Forest River or any other), and said they their experience is that it is considered normal. Just another data point for the conversation....
2016 Sunseeker MBS 2400R + Diesel Gen
2014 Honda CRV
Blue Ox Flat Tow Equipment
SMI Stay-In-Play Brake Controller