Mine was installed low down - close to the level of the bottom of the tank.
Interesting to read the stuff about airlocks though. When there is likely to be a freeze up, I cactuAlly run air through the system intentionally. I switch it on with the feed pipe removed from the tank and have the pipe at thew outlet end running back into the tank. This (mostly) empties the pipework of water. It certainly removes enough from it to prevent loads of water freezing withing the pump and cracking it. When I re-start (with feed pipe in obviously), there is a bit of spluttering for a little while but it soon flows OK. On my van mount I have a 100 PSI pump and half inch hose. No varistream needed as I have a "return to tank" setup. I don't know if this is why I don't suffer with airlocks but I suspect so.
On the very cold nights the pump may still fail to work the following day due to a little bit of water left in there but at least there is insufficient volume to crack it.
I had considered raising the pump so that it drains away from it on the very cold nights but feel that I might create more problems than I solve.