I suspect you have a bad heating element. Not enough to trip breaker or not function but on the verge and the emf's on the frame side (negative or ground) for DC circuits is causing issue with other equipment. Replace the heating element and let us know how you do.
BTW; welcome to irv2.com
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7