I have an early 2016 3160 (before they changed the look) and have been happy overall. I have had a few issues but not too bad overall. Toughest one was a exterior cooling fan connector for my fridge was disconnected and it took some work to diagnose and sliding out the repair (I eliminated multiple items before it went to the dealer) but was no parts, just they had to pull the fridge as the connector was buried in a wire bundle on the back of the fridge. My biggest pet peeve, as I do woodworking, is the trim pieces coming loose as the factory uses staple guns to install everything and on the thin trim, the staples go through the material too easy - easy fix, just annoying.
In part it comes down to how well the dealer prepped it and your pre-delivery inspection and also your expectation on what is an acceptable issue. Trailers take a pounding on the road (rolling earthquake is a good visual) so various minor repairs, adjustments, tightening things up, etc should be expected. Being handy and/or learning to do the minor stuff yourself reduces the frustration and time down as shops are typically always backed up during the summer.
As far as upgrades, I have done several substantial items including a solar power system, independent suspension, disc brakes, G series tires (yours have those already), slider tray in basement, USB Charging ports connected to battery, LEDs around and inside the island, LED bulbs for those that were still standard, and have a fan cover yet to install.
2016 Montana 3160RL w/MOR/Ryde IS and Disc Brakes
855W Solar System, TriStar MPPT-60 Controller, Magnum 2800W Inverter, 6 Trojan T-105REs
2015 Chevy Duramax SRW SB Crew Cab