Just completed a project to fit a length of trace heating cable under the van to the fresh water pipework and around the fresh water tank that may be of interest to the more competent diy ers.
After looking at many different ways to achieve some form of heating system I finally decided on a 12v trace heating cable. I had never come across this stuff before but it seemed ideal ….. more info about how it works at
http://www.ebay.co.uk/itm/390986887824
I contacted the company to ask a question to find they could supply 3m of this cable attached to a power cord via a thermostat which activates it below 3C.
I have installed it to run from a 240v power supply (we always use EHU in winter) to a 12v transformer located under the wardrobe through the floor beside the water inlet pipe to the thermostat under the van. Then along the water pipe to the tank and around the base of the tank.
Total cost of parts etc less than £50.
I put aluminium tape over the plastic pipes then taped the trace heating cable over that with more aluminium tape (this according to the supplier “converts” the plastic pipe into a metal pipe for the purpose of conducting heat) and then put pipe insulation over that. Did something similar around the base of the tank – not sure how effective that would be though. From experience the first thing to freeze up – quite quickly – is the water feed pipework on a cold evening – the tank takes quite a bit longer.
The cable I used gives out 12W/m at 0C and as the temperature drops the heat output increases quite a bit.
Tested at Chatsworth CCSite in December to -5C and compared to other vans that were obviously struggling with their water supply all seemed OK with our heater doing its job.
My Motorhome Mods .Online 
As predicted the front edges of the aluminium tape on the tank facing the forward air flow was coming loose. I have re positioned the tank part of the heating element to the front face of the tank so the edges of the tape are trailing and also physically clamped it in position with a length of aluminium angle held in position by the tank hangers.
Hopefully will solve the problem. Think in hindsight I would double up on the element in contact with the tank to give 24W/m – will see how I get on this coming winter season (not too soon I hope still want a bit more summer yet)
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