If you can't just leave it plugged in to 240v at the olds place then I would get one of those 2.5w - 5w portable solar battery chargers from supercheap and hook it direct to battery, throw panel on roof at a slight angle so crud washes off and you will find your battery will stay good for ages so long as it was fully charged in the beginning and disconnected from rest of van, these don't require a regulator just connect direct to battery........ mine have kept batteries charged that are higher A/H than detailed.....
Of course if your van has solar already then its not a drama .........
The other way is since your van battery is either GEL or AGM , when you drop your van off you just pull the battery out, take it home and throw it on a suitable multi stage charger to keep it sweet until next time, the charger just needs to be for your battery type and be about 5 stage so you can leave it to do its thing and forget about it, there quite a few out there that are not overly expensive things either........................... Being GEL or AGM it is quite safe to carry in your vehicle as they won't spill or vent nasty gas, just need to protect the terminals from stray contact with anything.......
If its not parked on concrete or hard gravel then I would sit the wheels on pads as well as have the draw bar on a stand also on a pad, put the jockey wheel under cover.......... a good spray underneath with surface spray can't hurt either, wheel covers are good or something that keeps the weather and sun off them... I would go a well tied down cover, may only last a year but if your not around then it will keep a lot of muck off it, they must be well secured with good straps or they do more damage than good...........
SCA 12V 4.8W Solar Maintenance Charger
www.supercheapauto.com.au