Most RV specs will show dry weight and also wet weight with propane and fresh water, but empty black/gray tanks. If not you can calculate wet weight by adding the weight for propane and 8# for every gallon of water.
Next you need to factor in the weight of any accessories and modifications. Those might include upgraded mattresses (foam mattresses can be really heavy), solar panels, generator, battery bank, etc.
For a family of 5, I would like to see at least an additional capacity of 2000#. Even 2 people can take up an added 1000# or so. The extra capacity is needed for cooking gear, food, tools, bedding, personal items and clothing, hobby stuff, books/maps/travel guides, lawn chairs, laptops/electronic devices, generator fuel, BBQ grill and fuel, picnic table, etc. You will need additional capacity if you are going to bring heavy items such as bikes and bike racks or even those sturdy zero gravity chairs.
Sadly many trailers are cheaply made, low quality and the manufacturer often skimps on even the absolute basics such as axle capacity and tires.