The brakes on my 2018 Navion were what I felt as "spongy". I gave them a pump or two and that was better. I took it to MB and they told me that when a motor home is built on the chassis, the manufacturer doesn't compensate for the increased weight. Their fix was to do a "super flush" of the brake fluid, change it out for a different fluid and readjust the brake pedal. That was the fix!! No more pumping the brakes and it stops. MB charged me for the service ($500) because it is not a warranty issue. I am very happy to spend the money and know the coach will stop.
Sprinter's have a reasonably long pedal throw on the brakes but they will stop on a dime. Most people interpret this long throw as "soft brakes" since their car pedal is high an tight. On a Sprinter just keep pushing a bit further and you will stop. No need to pump the brakes (unless you have air in the system).
Brake fluid is not going to tighten up the brake pedal unless you have air in the lines. A simple bleed (by any brake shop) would have cost you far less than $500.
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)