Let's just take a step back a bit. Firstly, charging from the van won't be an option whether you use a Bosch 12v charger or an inverter, because it'll flatten you van battery, then bothe vehicles will be stuck. 500wH is an awful lot to take out of any car-type battery, and bear in mind that your van battery is not meant to be discharged. Even if you had a separate deep-cycle battery (also called leisure battery), you should,'t discharge them past half-way. Driving the van around the block a few times won't be enough to charge it back up. Charging from a van or car is OK to top op your ebike battery as a one-off event, but won't work for charging every day.
Two batteries would work if you can find a friendly place where you can leave a battery on charge and call in once a day to swap them over. You'd need a mains timer socket as well because you wouldn't be able to rely on anyone else to switch off at the right time.
A generator is another option, but that might cause some annoyance running it every evening for 5 hours. Most likely someone will break into your van and steal it before long anyway.
Crank-drive bikes wouldn't be my recommendation. The bosch unit is nice and reliable, but 40 miles a day will soon see off the chain, sprockets and derailleur, which means time-out to fix them. Plus, all the gear-changing would become very tedious. There's plenty of good reliable hub-motored bikes that don't give problems with the drive train and you don't need to keep changing gear to keep the motor in its operating range. Crank-drive bikes are great for sporting use, but, for me, they just don't cut it for long commutes, touring or this type of use. H
Have a look at the KTMs with the Panasonic hub-motor, which are light, powerful and reliable.
http://e-motionevc.co.uk/store/product-info.php?pid298.html
All things considered, a twist and go 50cc moped would be cheaper even with tax and insurance.