Hi Dave,
The Yeti package includes some extra radiant insulating foil in the front and rear caps, along with 12V heat tape on the fresh water line in the underbelly, and 110V tank heat pads. There may be extra radiant insulating foil in the slideout floors, depending on what came standard in 2013.
Rather than being an orderable package, I think Four Seasons may be a marketing list describing the various standard and optional insulation values, Yeti pkg, dual pane windows and other things that contribute to cold-weather comfort. The 2013 BH 3610RE probably has all of that except the Yeti pkg and dual pane windows - although the previous owner might have ordered the windows.
Of all that, for camping below around 25 F, you'll want tank heating pads which can be installed as an aftermarket option. Probably cost about $750-800 in parts. I'd guess a dealer might want 4-6 hours of labor to take down the coroplast underbelly, install a circuit breaker and the pads and run the wiring.
We have an owner-written
Water Systems Winter Usage Guide that might be helpful to you.
