If it has been part-exed into a normal car garage, they will have no idea whatsoever about it. I got my first (Commer Wanderer) like that back in 1976. It's a good way to buy as they often don't know the value & are desperate to get it off the forcourt!
OP. Yo will only know if the charger is working (it won't be a "supercharger" cos they are used to boost engine performance & are mechanical blowers on the carb inlet) is to measure the voltage on the leisure
battery with the engine running & the switch on & off. Normal voltage should be around 12-13.4, with a charger connected it ought to rise to around 14.4.
Who did the conversion? There should be a plate somewhere, look then up on the interwebby for info & telephone them (if still in business) they may be able to sell you a copy of the handbooks etc.
Your logbook should have the name & address of the previous owner, contact them (if still at that address) be aware that someone may have died/seperated etc so there could be some angst associated wit memories of the van, but most of us have super memories of time spent on the road & wold be delighted to show off its capabilities.
Finally, find a meet to go to, there will be lots of helpful, interested & interesting people there, but be ready with plenty of tea, coffee & possibly something a little stronger in the evenings.:wacko:
Finally, enjoy learning what every thing does & try to write it down so you can pass the info on if & when you want to upgrade!