Hey Slick.............(
I've always wanted to say that
)
This is a pretty controversial issue that gets fairly heated at times. I don't have a Mountaineer, but I, and others, do have a rear hitch, and have caught some flak about it. It's do-able, but not without risk. Lippert ( the company that builds the frames ), says they will not warranty any frame that has been modified in any way, including rear hitches. However, Lippert only warranty's the frames to the initial purchaser, Keystone. Keystone, in turn warrantys the frame to the end user, and they do this on a claim by claim basis.
Frame flex is not the only concern, safety is also an issue. Towing two trailers requires greater driving skills, and an intensified awareness of everything around you. It is also illeagal in some states, and requires special drivers license endorsements in others.
There have been several posts on this topic that you can find using the search feature at the top of the page.