Do have a set of numbers which is just the truck no trailer?
Short answer is "truck with trailer on it" minus the "just truck" weight equals the hitch weight. There are a couple of other things to verify so I'll just walk through my set up so you can see the complete picture. Some people just look at the change in the rear axle weight but this is inaccurate. In the case of a TT some weight comes off the front axle or if the WD hitch is set weight is "moved" to the front axle. In the case of a 5er since the hitch is mounted an 1 to 2" in front of the rear axle. Some weight is transfered to the steer axle to ensure the front axle is not unloaded (think built in WD hitch).
Example:
My set up based on CAT Scale
Just my F350 with us and full of fuel.
Steer Axle: 4,560#
Drive Axle: 3,480#
Trailer Axle: N/A
Total: 8,040#
F350 with 5er.
Steer Axle: 4,840# (rated 4,850#)
Drive Axle: 5,420# (rated 8,250#)
Trailer Axle: 9,220# (rated 12,000#, 2x6,000# axles)
Total: 19,480# also know as Gross Combined Weight (GCVWR is 20,000# on my F350, What Ford says is the max weight for truck and trailer. Normally found in owners manual or published chart.)
GVW or "truck with trailer on it"
Steer Axle: 4,840# + Drive Axle: 5,420# = 10,260#
(GVWR 11,500#, What Ford says is the max weight of my truck can be. Found in driver's door jam. )
The 1st gross check it to ensure we did not overload the tow vehicle in my case my F350. We verify the GVWR and each axles rating to ensure we did not overload the Tow Vehicle. Looking at the combined weights. I'm close on my front axle but I've got lots of room on my drive axle. I'm use to running near max on steer axle as it helps keep the front end "planted" so it steers well. Just have to keep an eye on wear. I have room to spare on the drive axle. Diesel engines are heavy and tend to push you close to the front axle limit. I'm at 10,260# which is well below my 11,500# rating.
The 2nd gross check to ensure the overall combination is within the rating of the truck. So looking in my owners manual I see the truck is rated for 20,000# GCVWR or Gross Combined Vehicle Weight Rating. Since I'm at 19,480# I have about 500# to play with. So I'm good there. On a side note, if you look at a pre 2005 truck when the tow wars started all trucks topped out at 20,000# GCVWR. My truck is a 2004. Reading the charts, move from gas to diesel and it stayed at 20,000# newer trucks the GCVWR goes up. Switch to the towing rear end (3.73 to 4.10) and the 20,000# stays the same. In later model years the GCVWR goes up. In 2005 it just jumps from 20,000# to 23,000#. Since this rating is a function of not overloading the engine and transmission it's a warranty and performance issue. Since my rig goes through PA with the cruise set at 60 mph by dropping a gear I'm not worried about only having 500# to spare. The 20,000# use to put you near the limits of needing a commercial drivers license so the manufactures stayed under with their ratings. Now the laws in some states have changed and Federal is 26,000# so the truck ratings have climbed. Insurance has also changed from a 10,000# limit to 14,000# GVWR in some states. It's why the F250 or 2500's are so popular as they have a 10,000# GVWR, so they are cheaper to register and insure.
Now that the truck is good we need to verify the Trailer weight:
19,480# (total truck and 5er or GCVW) - 8,040# (unloaded truck weight) = 11,440# this is my trailer weight. The trailer is rated for 13,600# GVWR so I'm good here.
Looking at the weight ticket 9,220# spread over 2 6,000# axles so we are not over axle. Some people weight each trailer wheel to ensure no individual tire is overloaded. Since weight is not evenly balanced side to side (think kitchen on one side couch on the other kind of thing) it is an extra steep to prevent blowing tires. Works best with a set of portable scales. Some Jamborees will have someone to do this for a small fee.
Trailer tongue weight:
Truck with trailer on it 10,260# - Just truck 8,040# = 2,220# pin or hitch weight.
Pin percentage (of trailer weight) = 19.4% Trailer weight 10,260# / pin or hitch weight 2,220# = 19.4 percentage (0.194055)
A 5er averages around 20% pin weight and a TT around 15%. This is a balance issue to ensure the trailer tows properly.
Since these weight tickets I added the front AC Unit etc.. and I'm now closer to 22% or 2,600# pin weight.
Well that's pretty much all I know about this subject