Hi Matt,
The probe will have a couple of meters of wire on it & can be positioned anywhere in the van you like eg. in the spot you think will get coldest, or stuck to the piece of equipment you think is most vulnerable to damage from freezing. I wouldn't recommend putting the probe in the tank for the reason you've stated,.. it could use a lot of fuel before the probe would register a heat increase in such a large amount of water! The trigger temp is adjustable from 0 - 24C, so you can set it up to best suit your van.
The heater will raise the temp in the tank, but only very slightly. Most of the heat entering the van will be into the air from the heater itself & from your hose reel,.. it acts like a nice little radiator. If you un-reel your hose a little & spread it round the van a bit it would work even better. The heater shouldn't really be running more than 10 - 12 minutes an hour on average, unless the weather is very severe, or if you've set the trigger temp very high. The hot water going back into the tank isn't wasted though,.. even if it only increases the tank temp by 2 or 3C over the course of an entire night -- that heat distributed over 500 litres of water its bound to have a positive effect on keeping the van frost free.
The Fogwash has an auto ignition system. It senses water flow & ignites,.. as soon as water flow stops it cuts out.
