The info on the V5C will be a dead giveaway. If over 3.5 tonne, the taxation class with be PHGV rather than PLG (or similar). Also, the revenue weight should be given and is synonymous with MTPLM. Unladen weight is less than MIRO (which some converters confusingly call 'unladen weight') by at least the following:
- 75 kg for the driver;
- A tankful of fuel (say) 80 litres x approx 0.9 = 72 kg;
- A tankful of water (say) 100 kg;
- 2 x gas cylinders and 1 x leisure battery (say) 45 kg;
- All removable upholstery (say) 30 kg.
That lot comes to 322. So, your van with a MIRO of 2660 kg, probably has an unladen weight of no more than 2338 kg.
If you consider a 3.5 tonne van and only allow 300 kg for fuel, water, removable equipment etc. The payload would need to be less than 150 kg (3500-3050-300) for the vehicle to be subject to the lower speed limits. YMMV, but I don't know a van with a payload that low.