I don't know how others feel about this but even after reregistering the truck as a motor caravan I think you would do well to get it tested by VOSA as an HGV. There is just no way a regular garage could satisfactoraly test a large truck, and at 10t+ there is a lot of momentum, it is easy to cause a lot of damage in an accident. Testing a truck as a class IV IMO is just a way of avoiding the need to keep the vehicle in a good state of repair. If your truck is safe and relatively modern then you have nothing to fear from VOSA. In fact I always feel reassured comuing away from the VOSA test as they are so thorough that I know my truck is in good repair. Except perhaps the time they failed me for having the wrong type of bolts on the sideskirts and the marker boards 15cm too high!

If you have a non-EU truck, or a very old truck, testing as a Class IV is a good way of avoiding VOSA as in some cases it is impossible to get the vehicle through their test. Air over hydraulic brakes, or full time 6x6 can make it impossible to pass the HGV test.

My experience was very simple, although it may be different now. I spent 4 days on an Class C course and passed first time. I can now drive anything rigid (32t in the UK). I brought an 18t truck and when I went to change the tax class from good vehicle to Private good vehicle at the DVLA office, I asked if I could reregister the body type as a motor caravan. Two weeks later my V5c ame back with the body type ammended. It still doesn't meet all the requirements as the conversion is slow progress but I was never asked for proof.

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http://jimthetruck.blogspot.co.uk/