On the bottom's of shower floor it should be portable toilet also. (With two ussage failities. First for emergency when you have to sit down for longer. This option is not recommended because driver would need to dispose and clean it after several usage. But in emergency it would be fantastic. Also this option is very popular to instal in british camper world... second option is kind of pissuar thing or same toilete as above but with swith to collect urine liquid in separate container or simply slowly dispose it drop by drop slowly outside on the road, and after driver is back on the road this outlet could open faster to dispose urine liquid plus gray dirty watter from the sink and shower. In my experience it could be flash with glass of water to minimise urine smell to minimum. Also small sink should be installed in the middle of the cabin next to the driving wheel because this way it will be useful. Placing sink in the shower corner will make it unasable and useless. It could be and it should be hidden like a draw not folded.. middle of the truck has space for it already. I personally would replace your shower unit with camper toilet and install shower curtain where the passanger seat is located (plus with special hard easy to clean shower floor that look like any other floor). I would find the way to fold the seat some how (or use very basic simple version of seat not heavy and bulky). Because then you can fold this seat and make shower within two seconds.(english camper vans have this plastic or hard easy to clean floor in the bathroom-toilet fllor. Brilliant idea because this can be easy install in all trucks for minimum costs..) passanger seat is useless and problematic and uncomfortable for long distance drivers. It would be much better to install circulatet and recline driver seat that can rotate toward passenger seat and relax rather then use of incomfortable passager seat that wasting tiny space. My idea is better because you would have peace of mind and acces to all your belongings instantly... as folding both beds and remove four laguage bags just to make acces to the sink or shower would be nightmare on long run, where time is so valuable and drivers working hours extended to maximum. Last advice is to install trailer emergency sensors to alarm driver during trailer coupling that trailer is too close and last advice about better lights on the trailers back because it should be standard to install two in the back (each on the corner not the middle!) Plus two extras in the middle of the trailer (both middle sides with reversing additional lights). And camera for reversing as a standard. All this to avoid truck demmege during reversing.