What is a good weight for 35' fifth wheel?

I guess that it all depends on what you mean by a good weight. Here are some things to consider:

1. GVWR of the FW vs. Dry weight.
2. Hitch weight that your truck can handle so you still have enough capacity for cargo and people.
3. GCVWR - This is the gross combined vehicle weight rating. In other words, you don't want to exceed this total number with your tow vehicle, FW, all cargo, liquids (propane, gas or diesel, water, etc.), and people.
4. Fuel Economy - Typically, the heavier the FW, the lower your mileage. However, aerodynamics comes into play here also.
5. Axle Weight Ratings - You don't want to exceed the maximum rating for any of the axles in the TV and FW.

Typically, the aluminum frame FWs are lighter than wood frame construction. And yes, you're correct that many of the newer models are using newer materials to make an equivalent FW lighter than it would have been 10-15 years ago.

Our signature 5th, is a tad over 34' and its GVWR is for the sake of arguing, 14K. It is a 2009