I may be a gen1, but I've had my RTT on my X for several years with no issues. Do NOT use the crossbars to mount it and use the main rack tubing. While indeed it does say 125lb, I would agree to it being the dynamic load for safety/roll-overs, but it's been absolutely fine off-roading with it. I'm not sure if it's the same story repeating itself, but it may seem that maybe the Gen2's rack may want to come lose. I found the racking works that well, that I used the Frontier rack on my trailer, and that's been good for the last 2 years. With the 2 of us and the weight of the tent, that rack much be supporting a good 600lbs+
MPG loss with the RTT has been negligible for me. I've done some of my best tanks of fuel with it on, and also the worst. I think worse (along with the knock sensor issue on the gen1) would be more wind related, but I find the X efficiently inefficient.
For mounting, I just had to use longer bolts since the ones that came with the RTT weren't long enough.