I remember asking these same kinds of questions before I took delivery of the trailer.
On new trailers the power inlet is streetside, towards the rear. I believe the exact location does vary somewhat from one model to the next. Mine is within a foot or two of the rear bumper.
My cords are 25'. I have two, one 30A, one 50A; I use whichever matches the campground. I also carry in the trailer a 25', 30A extension, an adapter for using a 30A cord with a 20A outlet, and 25' and 100' 20A cords. I have available at home, 50' extensions for 30A and 50A, but they're heavy and big and I don't bring them unless I know I'll need them.
I have never been to a developed campground, open to the public, that had offered electricity but where a 25' cord would not work. I use the long cords when I'm parked in my driveway somewhere, and at ersatz "campgrounds" for special events. In my driveway, for example, I have a 50A outlet on one of the buildings where it was easy to install, but I usually park a little ways away from there and the cord won't quite reach.
Speaking with more experienced RVers, the usual answer I get is that they end up using the extension cord once a year at most. Often, it's not so much a matter of absolutely needing it as it is wanting to park in a particular spot where the cord won't quite reach (for reasons of wanting a better view or to leave room for a tent or something).
Finally, you're probably going to want to carry a 20A cord anyway, if for no other reason than that it's handy for power tools and the like, and in the rare cases where the outlet is more than 25' away you can use that with an adapter and just be careful not to run the A/C and the microwave at the same time.