it wont melt the tank mate i am going to put a 1 kw heater in mine plus i am going to cover it with 2 cheap double winter duvets ,
I am going to build a frame around my static tank and insulate that with loft insulation and cover with a waterproof cover ,
I think its important to keep water in static tank at a reasonable temp otherwise it will hard work for heater to get water in van mount tank to a reasonable temp,
MIKE