So I don't want to type out a huge diatribe so I'll keep is short and sweet. I'm tired of having to do other peoples work for them. I.E. I carry a full route and part of an aux route. Once I get done I have to go take off 30-45 min off other carriers that can't finish their own route. They won't request a route eval either, just want to hose the post office for as much OT as they can. Amazing how when the T6 or a CCA runs their route they find undertime on most days..... that's the main reason.
I hope you weren't misled into thinking this type of situation doesn't happen on the rural side.
But back to your vehicle question, my 02 Durango (V8) got 8 to 10 mpg in winter and 10 to 12 in summer on 80 to 130 mile routes. EMA covered gas and minor repairs. I started out in an 04 Chevy Venture, 25 mpg on the route in it.
My office had an RCA driving a quad cab Ram for a while. She was a short gal, under 5'5. I have no idea how she managed it. She never did come in at or under evaluation, usually at least an hour or 2 over.
Does your truck have a topper or tonneau cover of any sort? Depending on management in your office, you might get away with an open bed, but the mail and parcels are supposed to be secured and able to be locked in your vehicle.
Single cab, extended, or crew cab will also make a difference in whether or not your truck will work. It will also depend on the routes in your office. Higher volume more suburban type routes with lots of cul de sacs and tight turns might be a challenge.
PLEASE practice driving from the middle beforehand!!!! Find an empty mall or movie theater parking lot or go down a back country road and try it out. Pretend light poles are boxes, stop put your arm out, pretend to deliver. Park, scoot across get out, turn, back up, do all the things you will encounter on a route.
Since you were a CCA, you know how to handle mail so you have that as an advantage. If your office will let you, grab a tray or two, a flat tub or half tray, whatever the rurals you will be training with use and see how you might want to set up your truck. If one of the rurals at your office drives from the middle, watch how they load and ask for tips.
Rural is a whole different beast than city. Good luck!