If you are checking the voltage with a voltmeter, especially a digital volt meter, you won't see the variations that make "flicker". You need at least an old-school analog meter, or better an oscilloscope to see them. If your Webasto is like my Espar, the glowplug takes tens of amps when active, so there is a LOT of spiky current consumption on it.
You can minimize this by having the Webasto have a nice fat wire going directly to the house battery (through a dedicated fuse at the battery, of course), so that you have the least resistance, and the voltage drop to the Webasto isn't shared with the rest of the house systems. You *might* have luck with one of the very large capacitors from a car stereo place - you know, the capacitors the kidz with the Thunder-Thump3000 stereos use to try to get more phat bass[sic]. However, I haven't tried one, and when I run the numbers on stored energy vs. the energy demand of the heater it doesn't look good.