Not sure where you are in relation to Happier Camper
in Cali. but you might google them and make contact. Bet they know who does good work. They rent refurbished scamp/boler/Trills as well as having designed and now manufacturing the HC1
molded fiberglass camper.
Some campers floor plans will have a "place" for the bathroom even if it doesn't have that option, possibly a closet instead. Others will have floor plans that totally use up the space where a bathroom would go. Hard to tear out a dinette or couch and build a bathroom. Easier to install one in a closet that was where OEM was going to put one if ordered. So it depends.
RV bathrooms are not required to rent. And are somewhat complicated for novice campers, and then of course they have to be maintained. I would doubt that your renter is going to dump the tanks at the park without a learning curve so you may well get your camper back with sewage and gray water tank needing to be emptied. If you don't have a dump station at home then you would have to go to campground to dump after they bring camper back.
Also there is the ever popular if the feature doesn't exist no one gets all pissed off because it doesn't work right or because they can't make it work. E.G toilet won't flush because one of the kids left the light
and fan on and drained the battery
so the water pump won't go. Novice isn't going to think "dead battery" for toilet won't flush. No flush toilet = no toilet issues. No hot shower = no water heater / shower issues.
I would consider telling them they can use whatever facilities the park has. Folks that want modern toilets and showers will stay at those campgrounds, folks that want solitude and rustic probably already know how to make a pit toilet work :-)
In the small Scamps with front couch/bunk floor plan many people will slide a porta-pottie in the center floor cupboard. And those with closets may put a porta-pottie in them. This works for emergency or middle of the night use, or shorter stays at places with really nasty pit toilets. A porta-pottie tank can be dumped in a toilet or outhouse without hoses or dump station. You can do it when you get home in your own bathroom. Just something to consider.
You might go to the scamp
store and price out gray and black tank, drain valves for both, cost of PVC and vent stack for the tanks, and on demand water pump if your camper doesn't have one. Then toilet and sink, plus fixtures. Water heater too if you intend to add a shower. Water line and connectors won't be that large of an expense compared to the others. I built one 25 years ago in a school bus camper and using surplus parts for as much as possible it was still probably far and away the most expensive and difficult part of that bus to camper conversion. Always seemed to be a piece of frame right where I needed to make a hole, or no frame where I needed to secure something. I do sometimes miss the hot showers and flush toilet but 1/2 of that "joy" was because by god I worked hard to have it.