I have done it in the past when they often only had two points in the van. If you only want more sockets for lower wattage things like TV, sat receiver and charging things like the phone or laptop why not use a short extension sockets with 4 plug spaces?
David
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Depending on the age of your van, and the manufacturer, there may be warranty issues.
We have added 3 sockets to our present van and Swift's response was that if there was a problem that was traced to these additions then THAT PROBLEM would not be covered.
There was no question of it totally invalidating the warranty.
We also use a socket strip as David suggested, it is fixed behind a flap under the front chest and is ideal for charging batteries and phones.
In our really old caravan dh added more power points. In our new 'van we got the dealer to add one on the outside because i didn't want to go messing up the warrenty.
I've added sockets to all our vans in the past. As long as you're sensible and don't finish up with a dozen or so you'll be fine, but remember to keep an eye on the current - the more sockets you have, the more temptation to plug in another bit of kit! (Oh, and remember, there are sound rules about how to do it properly.)