I looked at both of those 5ers when I was shopping this year. Honestly they are both dually diesel territory. The FL without a doubt needs the dually as it has a very heavy pin weight. The RE also is heavy on the pin. I would not want to tow that much weight with a gassed and with a diesel you will likely be over payload on a single rear wheel..... to give you an idea, my SRW diesel had almost 4000 lbs payload to start but that quickly dropped to 3320 lbs once family, fuel, gear and pets were considered. My 5er was just under 11000 lbs dry weight and weighs 12380 lbs loaded with a pin weight of 2620 lbs. Honestly, my truck handles the trailer beautifully but I wouldn't want to go any bigger/heavier or have any less power. The exhaust brake is wonderful for stopping it and controlling it going down hills.
Just as nice as the Montana's but a little cheaper and lighter are the sabre 5ers. I have the 34REQS (rear entertainment, quad slide) and love it. You could stick to your SRW plan with this 5er and not sacrifice anything. And being cheaper, you could afford the diesel tow vehicle. Just another thought/route for you.
2014 Silverado 3500 Duramax, SRW, Crew Cab, 4WD
2014 Palomino Sabre 34REQS -
2011 Crossroads Zinger ZT26BL - sold in 2014