You asked "what cargo would weigh in a moderately loaded 36' fifth wheel" and it looks like you got replies all over the place but no real answers.
We sometimes travel for up to two months at a time and only average about 1200# of cargo in our TT on those extended excursions. People, pets, etc. do not contribute to TT or 5er cargo weight.
To cut down on cargo weight, we don't travel with a full fresh water tank or fully stock our panty or refrigerator until we reach a destination point where we will be staying for a week or longer. Nor do we fill our storage closet and draws to their capacity.