Roy's hit one nail on the head. Each vehicle is designed to take a certain amount of excess weight at the back, and still provide enough weight on the front end to keep the steering tires
in good contact with the pavement. I once used a friend's Ford Crown Victoria to haul my trailer (that i normally towed with a highly sprung F150)across a campground. The tongue weight was such that the front wheels were barely in contact with the ground, and the hitch was almost in contact with the ground.
The other issue is the design strength of the receiver hitch on the tow vehicle itself, particularly the vehicle receiver hitch attachment points and bolts. Some receiver hitches will have two numbers, with and without weight distributing hitches.
There are two aspects to interpreting tongue weight, the trailer's, and the tow vehicle's. Most home methods for calculating it are based on the trailer (using a bathroom weigh scale and 4" x 4", etc). Although this method give a good number, it isn't the whole story, especially with a weight distributing hitch. The best way to get a proper idea is as follows:
On your next trip out with the unit fully loaded, plan to stop at the local truck weigh station.
Weigh the entire train, tow vehicle & trailer, axle by axle.
Park the trailer nearby (most weigh scales have extensive pads), and with the same passengers in the vehicle, weigh the tow vehicle axle by axle.</blockquote>
This gives you all the numbers you need.
<blockquote>1. Check the tow vehicle's total weight in the first configuration, is it within the GVWR numbers on the driver's door. You can also check the weights axle by axle.
2. The total difference between the first and second configuration is the total loaded weight of the trailer. Compare that to the trailer axle GVWR limits. Also compare to the vehicle's tow limits and hitch calss rating.
3. Now, add together the tow vehicle's axle weights in the first configuration, and subtract the total weight of the second configuration. This is the trailer tongue weight, and the additional weight transferred to the tow vehicle. Compare this difference to the proper hitch rating.
4. The trailer tongue weight should be 10% to 15% of the total trailer weight in point 2 above.
If it is much more, the trailer is likely towing front down, and transferring too much of the trailer weight to the tow vehicle, and risking hitch damage on bumps. As well, yuou may be exceeding the hitch tongue weight rating, and risking breaking the hitch from the tow vehicle.
If it is much less, it is likely towing front up, giving poor towing and risking trailer bumper damage. On a really rough road, this could seriously damage the whole hitch assembly.</blockquote>
If you do not exceed the above limits, you are fine.
If you exceed the hitch tongue weight, and the trailer tongue weight is 15% or more of the trailer total weight, you are very likely towing in a front down configuration. Raise the ball height to transfer more of the trailer's weight to its own axles. Hopefully you have one of the adjustable bolt together ball mount units with two bolts, or you will have to purchase a new ball mount.
If the trailer is towing well and level, and you exceed the hitch tongue weight, try different trailer loading schemes, reducing what you put at the front of the unit, and loading either the back, or above the axles.
When I first started trailering many years ago, a wise man recommended this procedure, and it has never steered me wrong.