It is possible to design your interior so that all of your tanks are above the floor. You’ll have to give up some of your storage space to do it though. I’ve had truck campers that were built just like that.
Before TC’s started being built with basements, the standard location for the fresh water tank was above the floor, just below the cabover area inside because you needed a step up there anyway. The TC in my sig has a 30 gallon tank right there.
A previous TC of mine had the black tank sitting above the floor, directly below the toilet that was sitting in the wet bath shower pan, and was about 10 gallons IIRC. There was about a 6” step up into the shower. The grey water did NOT use that tank, and it would last about a week with one person using it, 4-5 days with two people. Our current TC has an 18 gallon black tank below the floor, and it will easily last a week with two people and have room to spare.
The grey water tanks may be more of a challenge if you want a shower in your trailer as well. There has to be a minimum amount of room below the pan for the drain and p-trap plumbing. You might think about using multiple 6” PVC pipes for your grey water tanks. They COULD be below the floor, and between the rails. They would be more durable than any standard RV holding tank as well.
:):)
