I use a stainless steel pot on the stove to heat larger quantities of water. I also have a 2 part roasting pan, enameled, and both the lid and the bottom can be used on top of the stove.
I have taken in rain water, (almost RO) and brought it to a boil. Then add the boiling hot water to a garbage can of room temperature rain water. Depends on how warm the tanks, and which garbage can I am filling, it might take 3-4 heatings.
Collect rain water via a pump from the tub in the yard to one of my water change prep cans. I have 3: 20 gal, 32 gal and 44 gal.
Use a smaller pump to fill (about 2/3 full) the roast pans on the stove (gas stove). The roast pans are large enough to span 2 burners. Bring the water to a boil, stirring occasionally.
Carry (VERY CAREFUL) the roast pan to the garbage can and dump it in, pan and all. The garbage cans are on wheels, so I can do this in the kitchen then roll it to the tank.
If the water in the can is not hot enough yet, pump more water out of the can and into the roast pans. Repeat until I have enough water, the right temperature, for water changes.
If I am just topping off the tanks I will not bother to heat the water. I bring in the rain water and allow it to come to room temperature, but no need to match the tank temperature when it is that small a volume.