I wouldn't fit any truckmount in a van with less than 3500kg GVW
Remember your weight bias won't be equal front/rear. You might be OK on one axle but overweight on the other. You won't find out until you're on that weighbridge and a fine is in the post.
Besides that, running a van up to its weight limit all the time will shorten the life of tyres, suspension, brakes etc. The same as the philosophy of having a bigger engine truckmount running at lower revs, if you're working well within maximum capacity you're wearing stuff out at a lesser rate.
It's also VERY dangerous to have a fully laden van, your stopping distance is going to be affected and the stability will be awful, lolloping round corners like it's going to roll over... you're much closer to the limits and more likely to end up in a crumpled mess.
Just my opinion for what it's worth
