The extra axle is called a Tag, with only 2 tires. I guess the chassis builder needed the extra tag axle for the weight.
I have a 36 ft. 94 Bounder on an OshKosh chassis with a rear diesel that only has a single rear axle.
The cost of 6 tires will vary and would be available at any good tire shop but expect to pay a minimum of $2000
Good Luck