I don't have data for the 1999, but the 2002 2nd generation 3500 had a GCWR of 21,500 lbs for the 245/505 Cummins HO/6 speed with 4.10 gears and 20,000 lbs for the 245/505 HO Cummins/6 speed with 3.54 gears - and that's with 4 wheel disc brakes. "Non-HO" Cummins trucks were lower. Therefore, even being very generous, it would probably be safe to assume that your 1999 has a GCWR of 20,000 lbs or less, most likely less.
But being generous, at 20,000 lbs GCWR, you're probably looking at a wet, loaded truck weight approaching 8,000 lbs. 20,000 - 8,000 leaves 12,000 lbs towing capacity, and a 12,000 lb 5th wheel would have a pin weight of ~2,400 lbs (20% of total weight). 8,000 lbs + 2,400 lbs = 10,400 lbs, right at your 10,500 GVWR limit.
Will it pull more? Yes. Will it pull more without exceeding one or more manufacturer's ratings? No. Will it pull a Mobile Suites? Unless you're looking at a very early, very short unit, I wouldn't. Our 2004 36' Mobile Suites has a GVWR of 16,000 lbs - newer ones are even heavier.
2016 Ram Longhorn 3500 Dually 4x4 CCLB, 385/900 Cummins, Aisin AS69RC, 4.10
2014.5 DRV Mobile Suites 38RSSA #6972
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