Hi Fonz,
Congrats on the Bighorn.
Here's a link to all the manuals for the Frigidaire residential refer and related items. I looked through them and while I found lots of info on the Auto Defrost feature, I saw nothing on disabling it. I will be turning mine back on tonight ahead of a trip we're taking to Pigeon Forge, TN next week. If I see anything in the menu on the front panel, I'll let you know - but I'm thinking you can't disable it. Not sure what your use-case is (travel many days without AC power, boondock for more than a day etc.) - but I've had great success with mine thus far.
By the way, some of us who own and use the residential refer are supplying content for a *User Guide for this refer. The neat things about user written guides is that its "real world" from actual real user experiences. Combined with mfr manuals and you're in a pretty good place. When you get your unit, your experiences can also contribute to the residential refer user guide.
* Disclaimer: The User Guides are not a Heartland RVs document. Content is supplied by Heartland product owners and edited/produced by a Heartland product owner.
As for the in-door water and ice dispenser, it's great! I run a two-stage (sediment, then charcoal) whole house water filtration system in the utility area (behind "the wall"). Then there's a filter in the refer itself. We drink the water and use the ice - where I've never done so with past refers/ice makers.