I have a similar set up to what you are describing. I carry 25gal fresh and 29gal black tank, no grey. I also have a diverter valve at shower drain to divert shower water to ground when acceptable. I mounted fresh tank underneath just behind axle. I needed the weight behind axle to help balance out tongue weight. It seems pretty safe as axle is lower than tank. Been a couple years now, no issues.
This is the black tank I used, the valve just clears the frame. Also no issues so far.
https://www.amazon.com/gp/product/B004R ... UTF8&psc=1The "Hepvo" trap works great and saves a lot of room over a P trap. You wont need to raise floor for shower plumbing and don't have to worry about trap drying out.
I chose to have a "wet bath" style like you would find in a truck camper or small RV, the shower and toilet are in the same pan, 3" drain for toilet, 1.25" for shower. Saves a lot of room, about 3 ft x 3 ft.
When you get to laying out your floorplan keep in mind weight and balance and tongue weight. This calculator will help.
https://www.engineersedge.com/calculato ... alance.htmRemember water is about 8 lbs/gal. I tried to set up so Harley was balance neutral so trailer was still balanced when not hauling bike.
Good Luck!
McDave