Hello Derek,
Which bottles are you referring to? - If its the gas bottles, then as I stated earlier it exactly the wrong thing to do. Without gas how do you heat the water to put into the hot water bottles (unless you have an EHU)!
I have had to resolve this type of problem many times with customers over the years, usually at the beginning of the season, but of note, the company I worked for was exhibiting at a caravan rally. The air temperature was about 7 or 8 degrees. We were called to three different caravans where the vanner complained that the gas appliances kept going out after a few minutes first thing in the morning. We found two caravanners who had used a hot water cylinder insulation jackets to put round their gas bottles in the belief it would stop them going cold. Another we found had lined their gas bottle locker with locker some Correx (Plastic corrugated cardboard) to stop small items falling out, but had also stuffed it full of tow ropes and other brick a brac which did a pretty good job of insulating the bottles. We measured the temperature of one of the insulated bottles and it was 7 degrees cooler than the external air!
In all cases the problem was resolved by removing the insulation and restoring the free air movement.
On a personal note I took a Norfolk Broads boat during the October Half terms a few years ago. The nights turned very frosty, and as the boat had a steel hull and the gas locker had good free flow of air, I was not surprised to find that we had no gas in the mornings.
We had no way of heating until the gas was restored. The solution was to use a bucket of river water and dowse the bottles (avoiding the regulator). That was enough to boil a kettle for hot drinks.
The following year I insisted on the boat being fitted with Propane.