You could always go with a shell for a Four Wheel Camper. Then, you could finish the inside as you wish. I don't think I'd like to overnight in small quarters without a closed in bathroom and vent to the outside. The portable toilet would be ok though. It depends on what bathroom facilities are available where you camp though.
I think the only camper that is supposed to be used with the tailgate down is an Alaskan. Otherwise, you can use 8-8.5 length on a typical shortbed. Some people go longer, but it depends on whether it is made for a shortbed or not no matter what length it is.