Have just registered my converted VW t5 in Spain, it has been a bit of a problem but it can be done. I couldn't of done it without a company in Costa Blanca, it was a mine field they were brilliant. So to all the doubting thomas's YES
What was the biggest hurdle? I'm looking at doing something similar although my Ducato home-conversion is already registered as a motorhome (Caravan) in Switzerland but would love to register it in Spain as its a real pain to have to take it back to Switzerland avery two years to get the MOT.
Make sure the vehicle document is changed from van to camper, total number of seats must be shown, a lot of ITV stations will not do it, the Itv in Orehuela will but the company I used were most helpful. Google Spanish number plates .com