Best option is to figure out how to get a 3-4" hose all the way to the back. If you just go a short distance with a 4" and then a reducer to 2", you won't really be better off.
Next best option would be to insulate the 2" hose, but I would ask: Where is the heat going now? If the hose is inside, even the leaking heat should help; if the hose is outside, then yes, you need to insulate it.
Otherwise, if you can get a small fan to push the air, that would help.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10