New to TT, Bit timid to tow ...?

DiskDoctr: We are looking at new 2014 vehicles. We had considered "gently used", but that when these are 2-3 year olds with under 30K miles, you only save $2-3K, so I'd prefer a new one (warranty, etc) for that small difference. The numbers I quoted above are for the 2014 models.

All: Please forgive my being too anal about it, but if I can play back what I think everyone is telling me ... The real issue on limits is the "payload" force, not the pulling force, per se. To determine this, I would add up all of the weight that is carried in the TV itself, then add the weight that the TT puts on the hitch, and this must be less than the TV's GVWR limit. If that understanding is correct, then does the following make sense?

TV Content
--4 Adult Passengers.......600#
--Full Tank Gas............120#
--Packed Stuff in car......150#
===============================
..............SUB-TOTAL....870#

Using Toyota 4Runner
--TV GVWR................ 6300#
--TV Curb Weight........ -4675#
--TV Installed Options... -200# (est, changed 2014-07-04)
--TV Content............. -870#
===============================
--Avail for Hitch Weight.. 555# <<<<<<< corrected


Using Nissan Pathfinder
--TV GVWR................ 5986#
--TV Curb Weight........ -4168#
--TV Installed Options... -200# (est, changed 2014-07-04)
--TV Content............. -870#
===============================
--Avail for Hitch Weight.. 748# <<<<<<<


Using Dodge Durango Limited V6
--TV GVWR................ 6500#
--TV Curb Weight........ -4838#
--TV Installed Options... -200# (est, changed 2014-07-04)
--TV Content............. -870#
===============================
--Avail for Hitch Weight.. 592# <<<<<<<


Using Dodge Durango Limited V8
--TV GVWR................ 7100#
--TV Curb Weight........ -5133#
--TV Installed Options... -200# (est, changed 2014-07-04)
--TV Content............. -870#
===============================
--Avail for Hitch Weight.. 897# <<<<<<<


If I am on the right track, then the order of these choices are Durango V8 (897), Pathfinder (748), Durango V6 (592), 4Runner (555), with each one *technically* meeting the specs for pulling the TT's I identified. Of course, this is only working on the "hard, cold numbers", not soft considerations such as whether it will handle well in wind gusts, etc.

Also, for my edification, doesn't "tongue weight" change as the incline of the road changes? Ie, the downward component of the gravitational force would change and therefore the tongue weight be increased/decreased on hills?

I'd appreciate any/all critique of the above calculations -- thanks!!

Craig