On a T6.1 the alternator will charge them to 100%. which is far more than a solar panel will do in a garage.
My advice would be to use the van a few times to see what power you are using & how quick it recharges when driving. Then decide if you need a different strategy.
Parked up for even a month the leisure batteries shouldn't drop below 90% if everything is switched off. A two hour drive would get you to 100%, use it for 3 or 4 days without moving the van & then drive 2 hours home & the batteries should be back to 100% when you park it up again.
