I certainly haven't had this problem, but I have a few thoughts for what they're worth--
First, have you had a big diesel pusher before or are you coming from gas? Reason I asked is it takes a little longer to build up torque than with a gas engine, but the torque is a lot more and lasts longer once you get going. So it's not as quick off the line as a gas engine, and that might be a contributing factor.
Second, is your turbo boosting as it should? Look at your boost guage on your Silverleaf and see if you build boost pressure relatively quickly as you push on the throttle. There's a little bit of turbo lag but it should build boost pressure relatively quickly, and you need that boost to get you going, especially if you're heavy.
Third, have you tried starting with the transmission not in economy mode? It won't upshift as fast when it's not in economy mode.
Finally, are you sure your transmission is starting if first gear? Look at the Silverleaf in "Drive" mode and see if you see a "6 1" and not a "6 2" or "6 3", so that it's starting in 1st and not 2nd or 3rd gear. As an alternative, manually shift down to 1st gear with the Allison selector and start out and see if it gets going faster until you upshift manually. That will give you a feeling of whether it's really getting all the way down to first gear.
Those are some ideas, which you've probably already checked out, but they're all I can think of at this time, short of some bad diesel that for some reason has a low cetane rating and may need a can of cetane boost to get it going.
2019 Winnebago 22M
2015 Jeep JK Rubicon
Former Owner, 2006 Alpine Coach 36MDDS
Former Owner, 2005 Tioga 31M and Arctic Fox 22GQ