What specifically do you find lacking? Our 2006 Navion 24H has very good brakes, but then I never really stress them that much. I leave a couple extra car lengths between me and the guy in front because I'm driving a much heavier vehicle than he is and I don't want to have to hit the brakes so hard when the time comes . . . . and then, I rarely go much faster than 65 - I bought it as a leisure time stress reliever and driving fast on crowded roads doesn't fit either of those categories. We don't have lots of hills around here like I assume you do, but I use engine braking to avoid riding the brakes (and heating them up which causes brake fade) whenever I find I'm needing to keep my foot on the pedal for longer than just a few seconds
The Sprinter chassis has huge rear brakes, larger than the fronts. I would not say it's under braked at all. If you're overloaded and riding the brakes going down hills and you're experiencing fade, maybe you could upgrade to a high performance set of pads when you need to change them
Don