I had a van with a pull out worktop and for what it must have cost to make i cant think why its not done now.Caanot help you as to where you can get one but years ago all our Swifts had a slide out piece that came out immediately below the worktop. They were really useful and would be even more so now that kitchen work areas seem to have got smaller over the years. Cost cutting I guess by the manufacturers put an end to them. I have seen DIY versions done to increase the length of the worktop but nothing that comes as a kit.
Andrew