Need help with comfortable towing weight and length.

You have the GCWR numbers already. Load everything up in the truck like you will normally travel and go to the scales. From there you can subtract scaled weight from GVWR. That is available payload. Or tongue weight for a TT. Also you can subtract the scaled weight from the GCWR. That is how much you have left over for a trailer. Keep a trailers GVWR under the last number and you should be OK on both numbers.