We have a built-in shower in our truck but almost never use it. We purchase fuel several times a week. With 50 or 75 gallons of purchased fuel, truck stops provide free showers that we use instead.
The bathroom is an RV-type toilet/shower room. It is the toilet that we would not want to be without. Black water is stored in a cassette that is serviced from outside the truck. It can be serviced at a dump station, RV campsite or carried inside and dumped into a flush toilet. In expediting, where you can end up anywhere at any time, that protability is better than full-blown RV systems that provide the luxury of a true flush toilet in the truck but require much more fresh and black water capacity and a dump station-only option for servicing it.
A porta-potty inside the truck is also a common option. It provides the same kind of portability. We like our setup because you don't have to carry the cassette in and out of the truck to service it. Servicing it from outside keeps it out of your living, sleeping and eating space. When we drove trucks without bathrooms, we relied exclusively on public bathrooms and did not use a porta-potty. We carried one for emergencies but were always able to avoid using it. Sometimes that meant driving 50 miles to find a bathroom but we really didn't want to be that up close and personal with porta-potty contents.
The truck bathroom also provides a convenient place to hang towels and our uniforms if we are wearing street clothes, or a set of street clothes if we are in uniform. We stow our shower bags there too. The shower is also a great place to hang dripping rain gear or drenched clothes after coming in from a rainy day.
Reminds me of the George Carlin "stuff" routine.