Hi Lance,
The issue with the Fox is that it has 4 wheels but is not a goods vehicle, which is what confuses the DVLA. The Department for Transport have advised people on several occasions to classify it as a Motor Tractor which is what we did with ours, as John suggests.
Registering a CVRT as a crawler is not an issue as it’s obviously got tracks so the DVLA staff can easily compute that!
Chris