Yep this thread wandered way off in the weeds.
Adding a built-in bath is probably not worth the trouble and expense unless one came across a really good deal on a rig that was built without that option but with the space available where bathroom would have gone. Some scamps have a bath or closet option. If you found one with the closet you could build by following design of same model with bath. Tank location, vents and plumbing etc. Still probably not worth the headache.
The best advice I would offer the OP is that a camper is a device to meet your needs and uses. Try to honestly and thoroughly identify your needs, wants and nice to have features based on how you plan to use it. It's a lot like buying a house, you know what you need or don't need. Purchase the rig you feel best meets those at the price you are willing to spend.
If you consider a toilet/shower a need find a rig with that feature. How you plan to use it, and how often determine the value you will get from the campers features. It's a balancing act. If camper does not meet your needs decently you won't use it. If you pay for features that you don't use or really care about it just adds cost. Me I would have an actual list in order to not be sucked into buying based on features that really don't matter but seem cool.
Think about it this way a $5,000 camper is equal to 25 days @ $100 for lodging plus $100 for meals. If on the other hand camper allows you to enjoy locations and experiences that you desire for the next 5 or 10 years, experiences that you would have missed if not for the camper you got a pretty good deal.
If you can use it a lot say due to being retired or have kids where a camper will make family vacations more affordable great. But both of those uses, full timing and family week-ends will have distinct needs. Your needs are also distinct to you and your situation.