Ian
Dustydog's advice is absolutely correct. Unless you tow an absolute lightweight tiddler the standard supplied scissor jacks aren't worth a light and can be an absolute B to get positioned when punctures drop the van, as well as being unstable if you aren't very careful.
Yes a trolley jack is heavier but you don't have to buy a workshop sized monster and then you may be able to carry it in the car. Mine goes nicely behind the driver's seat. So much quicker, less effort, easy to adjust by small amounts and can even be used to assist levelling on site, plus you can use the one jack on both tow vehicle and van, and it will still work when you change either. No contest.