Review my plumbing plan

[Daybreakbus]

Hello!! I've got a few questions from what I've found about plumbing so far.

My base: the last 4 or 5 feet of my bus will be acting as a wetroom so I don't have to worry about tracking mud through the skoolie to get to the bathroom to wash off. I can just come in from the back door and bam easy peasy. I'm gonna have 2 sinks(one bath one kitchen) and a 5 gal recirculating shower all in the back of my bus, no more than 5 feet from each other because the kitchen is butting up against the bathroom wall. I want a 100 gallon fresh water tank mounted under my bus(opinion?) Because I'll be living full time off grid so I want it to last as long as possible. I'll also have a 50-60 gal grey tank(opinion?) Mounted under the bus as well. I'm getting a composting toilet so I don't need to plumb that.

Both water tanks will be mounted in the very back, can a 2005 37 ft Thomas school bus handle all that weight in the back?

Since they're both under mounted, how/where do I put the water pump and accumulator?

I've found 4 options for recirculating showers so I'll be explaining them here. Please give me an idea as to which is the best/most feasible option. I'll probably flush it after each shower is over, so I don't store that recirculated water.
1) Uses a small sump pump, shower loop, 12v water pump with uv light/uv-c sanitiser, charcoal and string wound filter
2) Has a sand filter, activated charcoal then clean pvc tune with UV light. Backwash to flush/clean strainer @ shower drain and before pump inlet
3) Floor with 1/2in falling collection pan. Small sump pump pushing it through an online UV filter to shower head. Fill 3/4 gallon warm clean water + circulate with heat on. Drain 3/4 and use fresh 3/4 to rinse. There's a valve to switch drain to waste/recirculate.
4) Water from drain collects in 5 gal tank. Pump pulls water from shower tank + pushes it through 4 filters(20 microns, 5 microns, 1 micron, .5 microns) then UV light. Water than hits t joint to go to shower/water heater. There's a flushable sediment filter before the pump so it doesn't clog.

I want a tankless propane water heater, I don't think I have an opinion on whether it needs to be vented or not I don't really care. I plan to put it in the sink cabinet in the bathroom, any suggestions on other placements or an actual water heater? I don't know what brand/which one to get yet.

[Rucker]

Composting toilet: smart.

Tank sizing: Many folks here have immense tanks-they can chime in on what is reasonable/nice to have/essential. My shuttle bus design is for maximum four days boondocking and I have 40/40 fresh/gray after doing quite a bit of online searching on the topic. We'll see if that's enough.

Water weighs about 8 pounds per gallon, and 800 lbs seems like a lot of weight cantilevered over the rear wheels, but again, many people do that, so I can't help there. Shurflow pumps can handle about 6 feet of head (they are self-priming, apparently). So as long as the pump is not insanely far from the tank it seems like location is not a concern. I mounted my freshwater tank above the rear wheelwell inside the bus, so the pump and accumulator are lower than the tank and directly adjacent to it. I didn't want to worry about self-priming.

I read somewhere that there are boondocking locations where rangers check to see that your graywater tank is at least as large as your freshwater tank. I can't find the reference right now, but that might be a consideration.

I struggle with the utility/practicality of recirculating showers and never really considered doing one. I'm going by my camping experience-wear the same clothes every day and swim to shower. I'm sure I'll feel differently about it when I hop into that fancy and expensive mattress I bought for the bus, but I built a huge shower stall just in case. Most conversion videos I've seen show people use their shower for clothes storage, not bathing.

I picked one of the mid-grade propane water heaters. It MUST be vented and in fact I have not seen one on the market yet that can be installed without venting, despite what people may say. Mine is mounted in a wall cabinet between the sink and bathroom, and vents through the little slider window. I remove the vent pipe when travelling.

Yes you can install without venting, but that's a huge amount of moisture put in the air from the propane exhaust, and may contribute to condensation and mold issues down the road.

I don't think the exhaust is particularly hazardous. However, if you plan to have a truly well insulated bus, and put the water heater in a small and confined space like under a kitchen cabinet, you risk carbon monoxide poisoning. Lack of oxygen for combustion is what creates carbon monoxide.

My recommendation is to plan on venting the unit, period. Best practice is to have TWO carbon monoxide detectors for safety (in case one fails).