One thing that has just struck me.............
Before you permanently fix the immersion heater into the side of your tank, have you remembered that RO is very corrosive to metal? In time the RO will eat through the element exposing the electrics.
I have the same set up but luckily I use a 27' immersion heater going through the top of my 35' deep tank. When I dont need it I can take it out and put a bung through the hole to stop any splashes.