There was a period of time when a lot of manufacturers were making larger coaches without a tag. By the time the owner loaded them up with some basic items they were overweight. When the exemption for RV's came into play, all it did was provide a little extra CCC so that these large coaches were not overweight when they rolled off the show room floor.
Our friends have a 40' Camelot that measures over 41+ feet. It has a ton of storage because there is no loss of room for the tag axle. It can easily be overloaded.
In the last few years many coaches are seeing tag axles added. One of the reasons I bought my Newmar was because it's the only true 40' on the market with a tag.
Tag axles have many advantages, not just one or two. They add weight carrying capacity, extra braking, straighter tracking, eliminate porpoising and one that is often missed. They'll turn sharper than the same length coach without a tag. The reason being, is that the drive axle is pushed farther forward making a shorter wheelbase, without giving up the ride of a long wheelbase. The steerable tags turn in sharper.
Lastly, as one person asked, slides are generally smaller on Class A motor homes. The reason being, you have to be able to traverse the interior while underway. Your large TT's and fifth wheels are often impassable inside because the slides meet each other. Often their designed to let you get to the bathroom or refer, but not to the entire coach.