Does about 20 - 22 on average when towing. Once you're up to speed, there's no extra load on the engine.
also, check the state of your rear chassis where the towbar bolts to. Just make sure it is all solid.
^this, absolutely. If the chassis rails or captive fixings that hold the towbar on are rotten (and they do invisibly rot), there will be a soft crunch, and the back of the van will come off...
Trailer was made for the mini from a caravan chassis by myself. Made it as light as possible to be towed on the back of the bay eventually