I'm not sure the oil film will be a problem as the temperatures the 5kw diesel heaters can get up to don't exceed that of normal engine operating temperatures.
I've not got around to tackling the engine plumbing into the circuit yet, but may do if I have time when i'm home in March. I'll put a diagram together before I do it and circulate my plan. I was initially thinking of using shutoff valves, but beginning to think its added complexity, and another thing to break. Lever valves would be nice and reliable, but having to open bonnet every time might get old fast. Just seen that valve docka....that looks ideal, great find! Maybe a switch mounted in the cab would be handy after all!
I don't know why I didn't do it earlier, but your post made me think. Currently I only have the diesel heater plumbed to one coil. However, there is potentially double the heating area available if I put a y piece in and run through both coils.....it should reduce tank heating times quite dramatically. That i will certainly be doing when I'm home.
We have the 40l in our camper, and we (my wife) can sometimes run it out of hot using the shower. I know its too late to be saying it, but maybe 22l is on the small side?
As for tank size, we have around 400l of fresh water, and without being too careful it lasts 3-4 days. Having said that we both shower every day and wash up maybe twice a day. With a family of 4, i guess water consumption could be an issue. How many days are you hoping to go between fills?
Looking forward to seeing plenty of pics over the coming week!