I can understand a supplier charging that sort of figure as all though it's fairly simple job at the same time it takes a fair while to do especially if your doing the whole system. Some suppliers do window cleaning as well and when you think in terms of what they can earn window cleaning then they have to make a van fitting worth while. I was going to get a supplier to do mine and he was gonna charge a similar amount but the van came in early so I ended up doing it myself. Essentially your just drilling a few holes in the van floor and when armed with a tape measure, torch, friend and good pair of eyes you have to be stupid to drill in the wrong place.
Spreader plates are a pretty essential idea. Your probably looking at a weekend to transfer your system depending how involved it is, fixed reels,ro , water heaters ect, if it's very basic you could probably do it in a day.
Simon.