Hello Beta and
from Toronto
1) Post the payload from the sticker on the drivers door post that has "Tire and loading information" From that you will have the vehicle specific payload.
2) A portion of the weight on your truck will be transferred to the trailer axles once the Weight distribution hitch is engaged.
3) Your going to be close on the GCVWR of your combo. Not the actual number but at least the 20% reserve that should be kept for safety's sake.
I drive with little or no fluids while travelling. Enough fresh water for pee breaks and to wash hands etc. Too many horror stories about tanks letting go, chinese steel strapping (
)
There is no exact weight calculation you can come up with other than a camping trip ready rig taken over the scales. Weigh your truck empty, then weigh it ready to go on a trip. Same for the trailer. You will see axle and tongue weights fluctuate on the loaded unloaded and wdh stages.
The size and length of the trailer don't seem to be an issue. Is it used or new? Are the tires of an inferior brand? These are questions that need to be asked.
At least you're here trying to find out, that is half the battle. Just my opinion.