Fitting these is not as easy as you first think. First of all, the tank thickness means that only a few millimetres of thread comes through to the inside of the tank. Then, the mechanical flange will not fit as the thread stops way too short on the flange which is designed for a thin copper water tank, you need the round flange. The round flange will not tighten properly by hand so you will need a 2 1/4 inch open end spanner for the outside and something like an oil filter wrench to grip and hold the rounded flange on the inside. There will be no space for any of the supplied washers, the tank is too thick. You will need heat resistant silicon sealer because the normal water proof stuff peels off with the heat that the hot water creates. Movement will occur when the van is in transit, water will be sloshed around moving the element so make sure the element and flange are as tight as possible with the correct sealer.
I made an ass of my first attempt and these points are what I have learned. So far on the second go, all is good.
Good luck.