In addition to what Jeff says, a tag axle rig will also ride a bit better, because a side effect is to reduce fore & aft pitching when you cross RR tracks or bumps. They don't put tags on for that reason, but its a side benefit.
All that said, a tag on a 30 foot RV is simply to add weight capacity to the chassis. My guess would be these 30 footers you mention are built on "stretched" light truck chassis, possibly stretched in length and surely stretched in GVWR. When coach builders cannot get a hefty enough chassis for the size body they are adding, at a priced they feel is reasonable, they sometimes start with a lighter duty chassis and add a tag and perhaps extend its length too. This happened often in the late 80's and early 90's, when coach sizes were growing faster than the available chassis.