The numbers Newmar published are quite clear and as close to reliable as you can get without doing your own testing.
As stated in the brochure, UVW includes full fuel tank, all mechanical fluids, but not water, propane and options.
Add the NCC to the ULV (both approximations per Newmar) and you get your GVWR. Easy to do and no guess work, no need to worry about uninformed sales folks or misleading information.
From the 2020 brochure: