Hi! We've owned Peanut, our 1973 amerigo
FG16 for two years.
These weren't originally equipped with bathrooms, and the aftermarket add-ons over the years have proven to become the weakest link in the plywood flooring AND the "bathtub" fiberglass bottoms--the fiberglass is bent and wrinkled and convoluted in that area...And there' was nowhere originally to hang the gray and black tanks, because whatever you used was meant to lie on the ground beneath with a loose sewer hose running to it from a improvised and often unsatisfactory hook-up.
SO, Johnny M's custom installation may be your best bet. Fiberglass the heck out of it. But first, figure out where and how you'll do your holding tanks!
Are you sure you have enough room in the tiny front Putin for a bathroom? Our 1973 amerigo
Putin (our term for both the porta-a-potty we carry AND the front closet) is very small. what seemed like a possibility to begin with ended up being an impossibility once we got inside the closet and tried to move as if we were, um, pottying and showering.
Instead, we carry a pop-up privacy tent, a black 5-gallon bucket for solar
hot water with a plastic cup, a bath brush, a plastic soap on a rope container, and another bucket with a bucket seat on it for sitting on while we scoop water from the black bucket over ourselves. We've simplified it about as far as we can go.
We use the campground restrooms/showers where it works for us, we dn't shower if spilling water on the ground is prohibited, we use only basic soap, no other products (for simplicity's sake) and we do "Elvis Presley" washes when we cannot shower (that is, you use a basin of warm water and a washcloth and wash your body like you'd wash your face, standing on a towel and being a bit careful. We even cut our hair short to make washing our hair over a basin easier.
Well, YMMV! I hate to see the floor of an amerigo
cut into...after all it took us to fix the rotted-out floor of ours, the rot that started in the toilet hole cut by someone and never properly sealed because that area of the underbelly is so convoluted it's nearly impossible to figure out how to attach things. I guess I have to go with Johnny M. all customized hand-done fiberglass (isn't it all?) being very careful to seal it properly! I can't stress that enough. And leaked water will go into the "pontoon" areas under the floor and stay there, rotting it from above and below at the same time.
Shots of the aftermarket toilet hole showing the irregularity under that floor...the size of that closet--the angle of the walls; the floor is smaller than you may realize...
But, hey, it's your trailer! You may well be able to accomplish what others haven't.
Wishing you all the very best at it!
Amerigo--a pretty good trailer, and fairly rare.