When you say "backfed voltage"...do you mean that you reversed positive and ground on the battery? That would let the smoke out of lots of modules. How do you know only the gauges are affected? Not sure on yours, but my gauges are fed from a "computer" so, if the computer was damaged then the gauges might all not operate but the gauges might be fine.
If you mean you applied positive to a positive cable...then that typically wouldn't cause any problem, just energize the circuit when it should not be - but no big deal.
Before spending money on gunshotting parts, I would do a thorough diagnosis. And if that isn't in your wheelhouse, pay someone who has the background.
2002 Newmar Mountain Aire Limited 4370 w/ Spartan K2 and Cummins 500hp
ASE Master Certified (a long.....time ago...)