Hiya
PHGV is the taxation class. Which class you fit into depends on body type among others.
A motor caravan is a PHGV regardless of weight once over 3500kg.
You have to prove it covers all the DVLAs requirements of being a camper to them, if thats what you are aiming for? If the V5 says HGV then thats what you tax it as, tacho and plating req'd. You may as well be asking if you can tax it in the same class as a Prius, I don't think you're supposed to be able to choose yourself!

If you google DVLA requirements for a camper several forums have a list, but getting the info from the DVLA seems impossible. Send a photo of each of these features, plus overall extras perhaps, with the ammended V5 to the DVLA is all I did about 2 years ago, no hassle. Look on the SBMCC for more info too maybe?
There will be other catagories of type, but I don't know what they are. A race truck for motorcross weekends doesn't count as a camper since a MX bike in a big campers garage is not required for camping, so seems to count as goods, different V5 class, tacho etc required.
I looked at one ex electric board cherry picker that had a flat bed put on the back instead, classed as something which allowed PHGV taxation but couldn't put anything in the back, even a spare tyre according to the seller, but again no tacho required.
I don't know how long ago you passed your Cat C driving licence, but since its illegal for a Cat C learner to carry goods, the driving school's vehicle is PHGV and doesn't require a tacho. They do because they're probably second hand and used to "work", and if the school need to sell they can easily. As well as the fact you need to learn how to use the tacho to pass the test.
All as far as I know.
ps Any pics please?!?