Our non-slide 31-ft has sofa, dinette, walk-around queen (short 75" length). All that plus a pantry and a single "lounge" chair. It seems to take the "thirty foot" range to get that.
I haven't seen a survey but I'd guess many (if not most) of us do not weigh their RV's. And I'd venture that those who do, find them heavier than expected.
So, I think your only defense is to weigh the coach you're interested in. Even empty, you can get an idea what the weight distribution is. You can estimate by examination (position of tanks and storage areas) where weight will fall when you load for travel. Don't forget passengers!
But YES, some RV's are born at or over weight and some are also built with lopsided weight distribution. Rolling your candidate coach across a truck scale at least gives you a starting point.
I didn't weigh ours for several years. Found that it was pretty close to max. Luckily my tire pressure guesses were pretty close.