Hi Dean,
I guess from your question that you don't need to use the mover to get away from home, but presumably you might need it to locate your van when you arrive at the site - if not, then of course you never needed to buy one in the first place (don't, anybody else, let's get into that debate again). Therefore it is no good waiting till you get there to charge up the battery, it needs to be in good fettle before you leave.
The first question is whether you have anything on your van that drains the battery when it is in between trips. (We had a Compass Ralleye that had an alarm system that would drain the battery in about 6 weeks.) If not, then provided you are very regular in using the van and always have a hook-up on site you should be OK. But of course if you miss a few trips and the "every month" slides out to three or four months then you may well get to site and find you don't have the juice to move the van.
But even if you do let the timescale slip a bit, and you store your van at home, there is an easy answer that involves little additional expense or effort. Just buy an adaptor for your standard caravan mains lead from one of the accessory shops to allow you to plug it into a socket at home and wire up the van 24 hours before you plan to go away.
We have had our mover on two vans over six years and have found this works fine for us.
Cheers
Mike A