If you really want to start with a "blank slate" I would suggest you look at an EggCamper
kit. The kit is essentially a double wall fiberglass hull with no appliances.
All of the things you mentioned could be built into the kit. Since you want something that you could use in the winter you could insulate the water pipes and insulate the underside of the floor. You mentioned that you are alone so you could make the side dinette into a bunk area and build your "command console" into the rear dinette area.
Regarding your earlier question on installing some type of rack on the roof, I really wouldn't recommend it but, if you really want to, I'd suggest something like a Thule basket type rack. It could be attached through the roof with stainless steel hardware easily enough (include ss fender washers to spread the load). I would not recommend carrying a water bladder in the rack though. Too much weight
too high. A bad combination. Try to keep heavy stuff like the water tank, a/c, etc low and close to the axle
so you don't stress the frame or the hull as much. It also helps keep the trailer from "turning turtle" if you get in high winds.
I have done extensive camping in winter and have found that, if you properly set up your camper and campsite, you can live quite well even when the temp goes down to -20F or lower (that's as cold as it has been when I've been camping, not an absolute limit). Ventilation is a must to keep condensation down and you want to keep plenty of propane
on hand. I use 40lb tanks and keep at least 3 on hand. You will also want to keep the propane
tanks a little warmer since at some point the propane
will no longer boil off. I found that covering the tanks with heavy black plastic to be good for gathering heat during the day. And keep them off the ground with some crumbled plastic below them to keep the heat from leaching into the ground. If you will be boondocking
you will want to have some type of generator
cell panel to keep the battery
topped off since batteries don't like the cold, especially when they are discharged. Another suggestion is to wipe or spray silicon lube on the door and window gaskets. It keeps them from freezing to the door/window. If you attach some type of skirting around the base of your camper the floor will stay a lot warmer.
Well, there are a few ideas. I'm sure others on the forum will have other, better ideas.