so they fitted something and didnt test it to see if it worked 
? and thats ok !!!!!
i would be more worried about what and if it could do damage to the van circuits
running of van battery and running the risk of flat battery and keep replacing battery,paying £300 is not that dear in the long terms of things
I take your point on the garage, but they fitted as I requested and it didn't work. I'd still have to pay for the time and I'd have to pay the additional time working out where to go from where we are.
A leisure battery only last 18 months anyway so whether it's a van battery or a leisure battery makes no odds. The experience of the guys that use them on here is that there is no problem with doing so.
In light of the above I'd say that the sensible thing is to do nothing for now or try running off the van battery. The worst that can happen is the van
may one day need a jump start.