With a 20' box, having enough space for sleeping is a no brainer since the car isn't in there when you are. Adding a sink, shower and toilet? That's like a tiny house challenge. I mean, an F body is just over 16' so realistically 18' of that trailer is gone when the car is in there. I would suggest you're better off relying on the track for shower and bathroom and spend your efforts on insulation, a/c, quiet generator, and outdoor capable fridge (normal domestic fridges are not designed to cope with 100F ambients).