Reason dealer would NOT 'just bypass it' IS due to liability
The Atwood XT model uses a 160*F t-stat to heat water to higher temp then control output temp at130*F by use of that mixer valve.
Bypassing/eliminating mixer valve could easily end up scalding some unsuspecting person/child with 3rd degree burns due to 160*F water at faucets
How a shower mixer valve would be a backup is an mistaken idea.
And after bypassing mixer you can't just change out WH t-stat .....use different connections (which could be replaced provided one is aware!)
Most issues with mixing valve is clogging up with scale/calcium
They can be cleaned----remove/soak in CLR
And you do NOT have to use an OEM one........go to Home Depot/Lowes/ACE and purchase a 1/2" Honeywell Thermal Mixing valve----under $100