I have a Frontier crew cab 4X4 for a company work truck, trust me you DO NOT want to be pulling a 6000lb trailer with it! I tow a 4500lb enclosed trailer with it every once in a while and truthfully the Frontier really struggles on hills, its fine on flats, but hills kill it. I would try and stick with a trailer in the 5000lb range at most, you can find some very nice campers in that size now days.
TT-2013 Passport 3220BH
TV-2004 F150 FX4, not exactly stock...
with 2001 Kodiak K215: 2010-10, 2011-12
with 2012 Grey Wolf 26BH: 2012-19, 2013-24, 2014-11, 2015-6
with 2013 Passport: 2015-13, 2016-15 booked