2004 - 2005 Holiday Neptune 12v interior lights fluctuate

Incandescent or florescent or both? I don't know if any 12v circuit breakers...those are usually reserved for 120vac. The 12v system uses fuses to protect them and if one goes out....it's out. There's no in between. Sounds to me like you're having a problem with the transfer switch or converter. Are you connected to shore power? Does it do the same thing while on your generator? Have you measured your voltage?