So the volume on my Valveking has been fluctuating while I'm playing it, getting softer and softer and sometimes cutting out completely for 15 seconds. So my question is, could it be that it needs retubing (I suspect this since I've had the amp 2 years and havent changed the tubes)
Your amp definitely needs re-tubing. The problem is probably one of your preamp tubes.
Yes, needs retubing, exactly what mine have done when they die, but they were power amp tubes, not preamp tubes. It's highly unlikely it's the preamp anyway if it's pretty new, preamp tubes last a long time.