Its ok to feed all bathroom taps etc from the tanks (filters and UV will make it nice and pure) but I am in two minds as to whether I will drive the kitchen tap direct from mains. There are a few challenges here. The first is that the water system does not always have the required pressure and so kitchen tap may not always have water. The second thought is that it means the boat is not quite self-sufficient.
Maybe a further drinking water purifier under the sink would make me feel better. There are some good ones around.
The MDPE piping looks quite good. You can buy it in continuous 50m lengths which means that I can run water around the boat without joints (no joints, less leaks). So I may use quite a bit of the piping in the boat as I lay out longer water runs.
Then there is the issue of stagnation. I have found some good tank cleaning products (for drinking water) that will sterilise the tank and the piping. Simply dump in the fizzy tabs and leave it. Flush through. Job done. If I do that regularly (every year) and follow advice which is to regularly drain an refill the tanks then I think I will be ok.
Finally, research shows that using copper piping for the sewage would be fine. The reason it is not used these days is purely cost. But if you have it then its fine. Yea!