It is a first year tax, for the first owner. Along with the VAT. As with new cars, that hasn't stopped people buying those. Neither do I expect to repay that tax to the first owner if buying second-hand, even if only a few weeks old.
Intelligent dealers have their own ways around this, pre-registration, offsetting costs against tax, running them as demonstrators, buying the base vehicles at far lower prices than private individuals can negotiate, personal contract finance schemes etc.
Current thinking is explained here. Basically the motorhome manufacturers don't/can't/won't type approve their vehicles for emissions once they have converted them, so the default is, in the absence of any other information, to put them into the highest bracket when first registered, with the punitive gas-guzzler tax..
In subsequent years if registered as a motorcaravan then it's currently back to £260/year.
There can be no doubt that a heavily laden converted van will emit far more than the base vehicle such as an empty panel van. If I believed their figures the Euro 6 base version of my 5.99 metre Ducato running at 3.3 tonnes, should be doing 44.8 mpg and emitting 166 g/km CO2.
Since it's actually Euro 4 (more efficient probably but more particulates and NOx), and in real world use I'm lucky to do better than 30 mpg, which simple arithmetic says it's probably actually emitting at least 250 g/km. Which would put it in a very high tax bracket.
More precisely there are 722 g of carbon in a litre of diesel. If all that gets burned to CO2, that's an additional 1920 grams of oxygen added, totalling 2640 grams of CO2 per litre burned.
Hence my 30 mpg vehicle is in the real world, on a good day, emitting 251 g of CO2 per
litre (correction, km) The same as my quick and dirty estimate. And that's just a 3.3tonne panel van, not a big boxy coachbuilt job perhaps running well over plated weight, with lots of extra stuff inside, even if the manufacturers had been honest about the realistic payload in the first place.
So you can see why MotorCaravan manufacturers don't want to acknowledge that driving big heavy things around emits lots of CO2 and have it there on the V5 and don't want to type approve them
If they were subject to the same rules as cars, which they would be if properly tested, it might be even worse, anything costing more than £40,000 list price has to also pay an additional £325 per year for five more years.
So that could be £2135 first year. Plus 5x £325. Plus 5x £145 over the first six years which I reckon is £6620 or £1103/year averaged over that period.
See
https://www.gov.uk/vehicle-tax-rate-tables
If initially registered as a light commercial vehicle up to 3500 kg before and after conversion, the first year tax is probably much lower based on the manufacturers figures for the base van. Thence only £140/year.
Re-register it as a Motor Caravan after the conversion of a second hand, maybe just a "pre-registered" base vehicle and you might avoid much of this, I wouldn't know.
Just keep it registered as a van and save another £120/year, but then it wouldn't be a MotorCaravan.
That's how I understand it.