Your gasoline and water alone are nearly 1000 pounds. The dogs, bikes, bike rack, and people are probably about 500-600 pounds. 3500 pounds of accessories and stuff really doesn't seem excessively far out of line to me for a well-stocked motorhome; things tend to accumulate. There's also some weight in the inevitable dirt and whatnot that accumulate when you drive and use the RV, not all of which can easily be washed and rinsed off.
The sticker may or may not be accurate. Some manufacturers actually weigh each motorhome as it rolls off their assembly line (and put those actual weights on the sticker), I think, though probably not all.