Things to be aware about Keystone (I'm an owner):
1) The "nationwide" warranty is as nationwide as your local dealer. Non-purchase dealers will typically not do warranty work. Per their agreement with Keystone, they can't refuse (some do) - but mostly they'll just quote you "months" in terms of a warranty appointment.
2) Your warranty is for 1 year. A typical repair time, better than average, with an appointment, for my rig was about 30 days. It's also 30 days for multiple items, but it can be much longer. 3 repair trips can add up to a lot of time spent in the shop waiting on warranty.
3) Keystone will not ship you parts. They will not work with you. They will not provide you technical support. They will talk to you and refer you to a "dealer".
4) If your rig is over 12k lbs and has factory tires, assume you'll need to replace them.
5) Watch out for rust if your rig was built in the winter and transported.
I think Keystone builds decent quality (typically) rigs that don't cost as much as other brands, but part of that cost savings means you're putting up with some of the service issues above.
Many of these rigs come down the same line - so likely you may want to ask not only Montana owners, but other 5th wheel owners what issues they've had.