Just to play devil's advocate then...
Is a higher capacity battery for the mobility scooter an option? If so, that would not only extend the range of the scooter but also put the extra power where it's actually required.
If not, then how about a power bank? Could potentially have other uses and is more flexible than adding a battery to the van and again could be carried on the mobility scooter in order to extend the range or to allow charging while not at the van.
Just thinking out loud really.
DIY of the panels is entirely possible, but I'd advise against it if you aren't confident. None of it is terribly complex but if you get it wrong you can damage your MPPT controller and worse still (IMO) end up with water leaking through your roof...
We've looked at upgrading the mobility scooter battery but sadly not possible, I'd have to have one built.
Power bank I've investigated before and not something I want to invest in to be honest, something else for me to lug about and hurt my back which I cannot risk, plus somewhere to store etc etc, I've ruled them out.
But are you stationary on UK Aires or pub car parks for your existing set up to be inadequate? If not you wouldn't need to put the panel out!
True, though then I have a panel I have to find somewhere to store / move etc more of the time than not, so if solars the way to go I'd rather "fit and forget" I think.