Just a daft question. But why do you want one when you already have an oven in the van, I assume? However, you, of course may be like me and have a EU van where ovens are not fitted. Great, no rattles and no rust. Plus extra storage space.
Answers could be that you can use the site electricity and not your own gas. Also it could be used for small heat up jobs but depending on the type of oven, and wattage, then sometimes the cooking ability is a bit optimistic on some models. They tend to localise the heat so you get hot and cooler bits. There is no fan as far as I am aware.
We have indeed looked at them as we do not have an oven, but the cons are the extra weight, even though it is only a few kilos. The extra space it will occupy and what would we actually use it for? If I wish roast chicken I buy a rotisserie one from a supermarket and I doubt if I would ever cook a joint of meat in the van. I am trying to eat more healthily so pies etc are not on the menu. We did have a microwave for a while which was quite handy for take away meals etc, but after using it only about 3 times in a season, we decided it was not worth the bother. It is now in its box in the attic.
I sometimes feel that they are a bit like many gadgets I con myself into buying at Shows. They look good and I see a use for them, but actuality proves otherwise. If you are a great rallyer in coler weather and feel that oven meals are your wish, then have a look, but again if you already have one with the van, why bother?