We have had 2 Montana fivers, the first one we purchased used and the current one we purchased new. First one was 38' and this one is just over 39'. I have a 2012 F350 SRW. I have weighed recently at Cat scale and I am not over. Don't really want to get the SRW vs. dually thing going. Bottom line is you need to be within weight specs.
Generally speaking, if you are seeing MSRP for a new Montana at $80,000 you can probably actually purchase it for closer to $60,000 or $65,000. I would suggest visiting RV shows and dealers. We determined brand and floor plan first, then began trying to find out what actual cost would be.
