I agree with Martyn in what he says. Most people are not as good at reversing as they are at forward driving, and with a huge "lump" on the back, and more so with twin axle vans, you are asking far too much of the clutch,and fixing your revs to avoid stalling, and slipping the clutch causes it toburn,hence the smell.
A simple but similar experiment can be done athome, with an angle grinder and a piece of wood, fix the wood in a vice, start the angle grinder and bring it flat disc side down on to the wood, gently, it will only be seconds before you have smoke,and a few more before fire,,,,,well, thats whats happening to your clutch.
You are bringing a stationary friction plate into contact with a metal disc, spinning at 1000 to 1500 revs per minute,,,,SOMETHING has to give,,and its not going to be the flywheel, BUT,,keep doing it, slipping the clutch, and over time you WILL end up with heat stress fractures in the flywheel,,and if that disintegrates,its good bye to youe gearbox, bellhousing, clutch and anything else the hunks of metal hit.