Second the AMT-12 if you already have solar fitted on the leisure side.
For direct maintenance I believe 50mA is the threshold for a fault code for the van's ECUs drawing too much current when fully asleep, so using that as a worst case that's potentially 1.2Ah (24 x 0.05) of capacity from the battery over a day.
In watt hours that's 1.2 x 12 so near enough 15wh
So your solar panel needs to be able to replace that at the same rate. As a rough assumption assume you get a good rate for a quarter of the day (6 hours charging, 18 not charging) then you need 15 x 4 = 60w
So the 10/20w panels are likely to fall short.
For reference a normal 5mm LED will generally run at about 20mA - so the load use in the calculation is about 3 LEDs - not that much.