Quilts will only keep in heat that's already there... not much good if you put them on something cold. Further to that, if you cover something cold with a quilt then put a heater in the van, the quilt will stop it getting warm!!
Antifreeze..... life's too short (and mornings too cold in winter) to spend an extra 30 mins at night and an extra 30 mins in the morning running it through and then thoroughly rinsing everything out again!! Mug's game :
Sure if the machine is going to stand idle with a heater but nobody to keep a check on the heater day to day, then the antifreeze might be a good bit of insurance.
Heater all the way - easy and relatively cheap. I use an 800w oil filled rad, keeps the air temp in the back of my van at around 16-20 degs depending on outside temp. I was running it lower than that but one of my female QDs on the TM's solution outlet must have frozen and it drips now.
It's right at the outside edge of the van, and the TM stands between it and the heater so must have got cold there. I run the heater up almost full now, nice and toasty
My van is insulated with fibreglass stuff like you have in the loft, on frosty mornings the roof is covered in frost apart from tiny little melted patches where heat gets out because of the roof beams