This thread seems to be dormant, but since this sort of thing gets found when people search for information, I thought I should add my bit.
Way back on page 1 or 2, dating from 2012 or so, someone put a link to my Swaraj Mazda motorhome.
I built that in the late 90's; it got too heavy so I added suspension and turbocharger.
That's 2 entire subjects; also a little side engine with generator and AC.
But all that info is common to the rest of the planet and can be easily found.
What I want to share here is my registration; I bought the chassis as a commercial vehicle, but registered it as a private car.
Look up the legal definition of a private car in India; while each state has their own rules, in practice I think all have adopted the india 1988 road rules. I'm working from memory here, but a private car has not more than 9 seats including driver, does not carry passengers or cargo for hire or reward, and that's about it.
Nothing says how many wheels it has, or the weight, or the size.
Now obviously, your local RTO officer is the one who has to accept this before you can proceed. Many states probably have a precedent set [Goa, Maharastra for sure] by an earlier registered vehicle.
I have no doubt most members will know how to deal with RTO officers. The point is, it's NOT illegal to register such a vehicle as private.
It's all really, really expensive; but many have built campers in India, both foreigners and locals.
I was last in India in 2006 and have no plans to return; but the place is still in my bones.