Here is a link to a map of POR:
http://www.wdwinfo.com/resortmaps/port-riverside.htm
You can sort of see for yourself where the closest buildings are...
I've not had much luck with requests, myself... we were in building 17 one year and in building 27 the next. Let me tell you, they don't LOOK that different on the map, but they felt WAY different. Maybe it's because there's really a difference, or maybe it's because when you go from 17 to the main building, it's like you're wandering through this forest and don't really know exactly where you're going. From 27, on the other hand, you walk out of your room, get to the "river" side (if you don't have a water view room), and you can see the building right away. I still wonder if it's a kind of psychological thing where you're out in the open by the river and can see the building VS. finding your way through the "forest" to finally get to the building....
I think there is physically a little more walking from 17, but I don't think it's as much as it seemed...
Also, if you can get your hands on an Unofficial Guide, it has suggestions about which buildings are best. Obviously, 14,15, 18, and 90 are all pretty close, but you have better luck than me if you manage to get one...
On the trundle bed... if you have 5 people, you will ONLY get a room with trundle. Their system is smart enough to not book you into a room that would not accomodate the number of people. f you only have 4, you would need to request a trundle room. As far as I know, all rooms in Alligator Bayou have trundles and all the rooms in the mansion section do not.