A few TC questions about host trucks and factory add-ons

I'm not even sure of what I'm looking at there. LOL To empty the poop tank I take it?

The Douglass and Sherptek bodies...I'm assuming those add LESS weight than a standard pick up bed?

I'm really curious as to the advantages of a diesel engine over a gas in a TC application. It makes sense to me if towing, but for a camper? As I'm thinking of full-timing, the longevity of the motor appeals to me, but I'm not sure if it's THAT important if I'm only driving a couple hundred miles a month.