OK, I pulled up the manual on the generator and if you are getting airconditioning we can eliminate the Generator and pigtail.
Now you have reset the breakers and the GFCI? Not just "checked them visually" but actually turned them off and then back on? A tripped breaker can "look" on but be tripped "off."
Is the AC that "works" the front AC or the back AC? One might be on one leg of the 50 amp panel and the other on the other leg (the one that does not work). A photo of your breaker panel mght help me too.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW