You could do it, but you would need to do a couple of trips to get water. The mondeo wont carry many 25L containers unless you have them all dotted about the seats. That wouldnt be too safe though.
I had a VW sharan people carrier and had 7 x 25L containers and the backpack in the boot, well away from me !
you can easy carry 8 25 ltr containers and a backpack in the boot of a mondeo estate my mates been doing this daily for around 18 months .
for £100 the wcw backpack is a good buy as it will able you to do most jobs the downside is you are having to refill after each house .
shurflo are allso a good backpack and 1 fill can get you around 3 houses as you get a better control of the water flow .
after having and using both wcw & shurflo i would go for shurflo , this is in no way saying wcw pack isnt any good as its a good tool to have and will be fine for most users needs .