For the experienced traveler India (being the huge country that it is) has great potential for exploration by campervan but you would need at least 6 months to do it justice. Tourists/travelers can buy any vehicle in India with a signed affidavit from a magistrate. It is probably best to research campervans and buy cars online once in the country. Vans and motorbikes are also easy to source. The paperwork to purchase a vehicle usually takes about 7-10 days and a dealer can help you with this. The main question is, are you up for the challenge?
Most travelers would not even consider hiring a campervan or purchasing a vehicle in India but traveling through India is not a journey for the faint hearted regardless of method of transport. Western campsites are non existent but many safe spots that you can camp for free. Guest houses are so cheap that you can always consider this as an option for resting your head if you could not find a suitable spot for your campervan. Obviously there are risks and dangers wherever you are in the world but India is generally a safe place for tourists in regards to crime as the majority of people are trustworthy Hindus who believe in Karma. It would be advisable to travel in a small group to reduce vulnerability.
Roads are generally in poor condition. There are some tarmac highways of increasing quality but these often get congested and need essential maintenance. Dirt roads and single track roads are more the norm so maybe consider a 4WD if your budget allows, although these can be difficult to source in India. Very early travel would also be advised in cities to cover more distance, as would paying a tuk tuk driver to navigate you to your destination. Road signs are few and far between so invest in a good map and allow a flexible plan with room for error.
The rewards in the way of spectacular scenery, sites and immersion in culture would make the challenge worth it but obviously it is no mean feat to travel around India in a campervan otherwise it would be more popular. Road safety standards and driving skills are similar to any other developing country. Beeping horns, over taking on blind bends and barging other drivers out of the way are the norm as are potholes, cows on the road and other obstacles. It may take a while to get used to things and gain enough confidence to get behind the wheel.
As you can see there are many reasons not to take to the wheel in India but for the brave few that do the experience was surely worth it!