I have Emerson , fitted it to my 500 Ltr some years ago, I was one of the first to do it and it works fine. You need to fit it about 10" from the base of your tank away from the sides (central) if you can.
Get a good thick cable to take the amps, the Emerson I use is only about 12" /14"" long, but low down.
You need a sealant called nail & seal its white and works fantastic cost about £6. B&Q
I fitted an out side plug the type they use with caravans , stick a timer on it so it comes on about 5am and by the time you start it will be lovely and warm.
Keeps everything warm in the van too.
Hope this helps.
Try and get the right hole saw, the blokes at the plumb centre will tell you the exact size you need, get as tight a fit as you can, no good with IBC tanks the plastic is too thin.
Two man job