Our first van was a 2004 Autoquest 100 so a bit different but apparently similar as far as the essentials of this thread go.
The Autoquest was wired to top up the leisure battery from the alternator/starter battery when driving but had no circuitry for topping up the starter battery at all. During the first winter we had the Autoquest the starter battery was flattened by the alarm (and possibly not switching the radio off completely). I used a Tronic T4X charger (from Lidl) on the starter battery from time to time in winter when it tended to run down. I could attach the crocodile clips to the battery and plug the unit into one of the van sockets, power to the van being supplied from a socket in our garage via the hook-up lead. The Tronic (like its Aldi equivalent and the rather dearer Ctek models) is a smart charger which should not damage a battery. I think the warning about disconnecting the battery is aimed at use of a basic charger which might cause problems.
Subsequently I fitted a Battery Charge Manager (BCM 12) from CAK Tanks which kept the starter battery topped up, in addition to the leisure battery, when on mains hook-up or when the solar panel was in use.
I think Lidl and Aldi both had their smart chargers as specials recently so with luck you might find one left at your local branch.
The VEH/AUX switch is purely for supplying power to the 12v habitation circuits and is redundant when it comes to charging. The warning about using it on VEH sparingly is correct, as we found when we inadvertently switched it over by mistake
Graham