Really sorry to hear of your woes! That is unfortunate, as our 2007 3400RL Montana was a wonderful unit with only a sink crack to fix with 7 years of use.
I actually haven't heard of frame issues on the later years, but I'm sure they are out there. Maybe more rare than before (not good in 2000-2006). I would exhaust every avenue - further negotiation with Keystone, lawyer, hot-line. At some point you will have to decide if the cost is worth the gain, and either just pay to have someone fix it right, or trade it off.