Take your
Trillium 1500 to a scale. Put it on the scale without the tow vehicle. We have weighed all of our campers at least once. We take to a landscape supply company that has a certified scale. They charge $10 and provide us with a weight slip. (Also record what trailer items were included/not included in the weight.)
Some people suggest that in addition, they read the weight of just the wheels resting on the scale (tongue still attached to the tow vehicle). That way they can calculate the tongue weight.
EDIT: If you are purchasing a camper, the seller should be happy to do this for you.