There is one length for a cb antenna (Really two but...) that is 9 feet. However it can be sprial wound so it's physically shorter.
As to mounting.. Depends on the coach., Spring is not usualy needed with the sprial wound fiberglass whips but.. May be if it's in a place where tree limbs can wack it.
On my coach the CB radio antenna is jammed in between teh couch and the wall, It's a fiberglass and plywood wall so electronically it's the antenna is basically in open air. SWR is 1:1 (Or so close to it I Can not tell the difference) and it works as well as I expect a CB to work (Which is not that good by my other standards) But If I can see you, I can talk to you.
(My other standards.. Well. let's see, from where Im parked now (South Carolina) I"ve worked California, Texas, Canada, Germany, France, England, Cuba, Scotland, Grand Cayman Islands and a bunch of countries I can't even spell.. But that's NOT CB)
Home is where I park it!