I would be very cautious about running an engine merely to charge up the battery.
When you start any engine from cold there is a automatic mechanism for supplying an enriched fuel supply to assist starting. The excess fuel washes away the lubricant in the upper part of the cylinder. This is not a problem with 'normal' cold start and running because it only happens for a few minutes, then the vehicle is put under load and it warms up quickly. Constant cold starting and the engine left to tickover will introduce this excessive fuel to a proportionately greater part of the engines running time and so shorten its life...
If you can't get a 240volt supply to the 'van then I would plump for a Solar Panel.
The early bird catches the worm but the second mouse gets the cheese.
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