Our Dutch import van has a battery fitted purely for the mover. It was an 85ah which was fine if we didn’t need to do a lot of manoeuvring between charges as there isn’t a charging system in our van. We’ve upgraded now to a 110ah battery which is a lot heavier. Will you need to move the van a long way, up a slope or a tricky access? If so you may need the bigger battery, if not the 85ah should be fine. We carry a smart battery charger with us and just put the battery on charge the day before we leave site to keep it topped up for the next site move. This option is a lot cheaper than having a charging system fitted to the van.
Make sure you have a battery box fitted to secure the battery and I would avoid a lead acid “wet” battery because of the possibility of fumes, a Gel or AGM would be better. Lithium is an option if you have deep pockets.