The GVWR is different from what you can TOW.
It tell you how much stuff you can LOAD.
My truck has a "registration" weight of 5828, a "wet" weight of 6700 and a GVWR of 8800.
My trailer has a 1300 pound hitch weight when loaded with water, stuff and etc. and a "wet" weight of 12100.
Subtracting the truck's "wet" weight from the GCVWR (Gross Combined Vehicle Weight Rating) of 20,000 says I can safely "tow" a trailer weighing up to 13,700# - so I'm 1,600# "light" with the trailer.
My hitch weight of 1100 becomes part of the "load" and is factored into the GVWR. Subtracting the truck's "wet" weight from the GVWR of 8800 says the maximum safe "load" is 2100# - so I'm 900# "light" with my trailer's load.
Both GCVWR and GVWR need to be considered.
Regarding service intervals: I change Mobil One at 3000 mile intervals when towing and 5000 mile intervals when running aroung without a load. - and in between those figures with combined driving - leaning one way or the other depending on how much or how little towing I've done.
My rig is a 5th wheel, so I cannot answer your questions regarding "load leveling" hitches, although my father and father-in-law both swear by theirs when towing their trailers.
Good Luck - and stay within limits for safety's sake. Maybe I'll see you out there some day. See my rig by following links in my signature.
1999 F-250 XLT Super Duty Crew Cab Long Bed
V-10, Automatic 4R100, 4.30 Limited Slip.
GVW = 6700# Towing Capacity = 13,300#
4 Wheel ABS disks + Reese 15K hitch
Dark Hunter Green. Ford Running Boards.
Grey Rhino-Liner. Captain's Chairs. CD+Tape
Polished Stainless Steel Wheel-well Molding http://members.home.net/jchartier/jnctrk.html