Dumb acceleration and braking questions from a newbie

[Rufus Scout]

My 1985 Marathon with Detroit Diesel 8v92 and 165,000 miles blows a cloud of black smoke upon accelerating from a dead stop. The cloud dissipates within a few seconds of acceleration and it does not smoke while motoring along. Normal? Problem?

Also, the throttle pedal is quite stiff, it really takes some serious effort to press on it.

As for brakes, I realize that we're talking about a heavy vehicle. It seems to brake from speed quite well, but the last segment of braking, say 20mph to a stop seems to be quite leisurely. Normal? Problem? It does not seem dangerous, but it requires planning for sure.

While I am not a complete motorhome newbie, my experience has all been with gas powered 2x4/plastic motorhomes, so I am curious. Would someone care to help educate me? Thanks.