Don't know for certain, but would expect the Duramax Colorado/Canyon to use the same T case as the gassers, so should still have a N position an be flat towable.
What problem with the battery disconnect? Installed one, use it when I tow, no issues.
2013 Thor A.C.E. 29.2
SeaDoo Speedster 150 HO or 2015 Chevy Colorado Z71 toad, depending on season.