Here's an update on my AC voltage issue using Chris' setup for those that might have a similar issue (the problem was 10%+ AC voltage sags and surges). It has now improved considerably (down to about 2 v +-).
I got ahold of an analog voltmeter and performed the test between the battery terminals and the inverter lugs that Chris suggested and applied a very heavy load. I could not see any difference in DC voltage between the two points. I general, I haven't had this level of voltage drop with any other load, so I figured it was likely something with the PWM function and the SSR.
Just as an experiment, I tried increasing the width setting on the Classic from 1.0 v to 3.0, v and noticed an immediate improvement in the degree of the voltage sag (this was immediately audible in the character of the transformer hum on the inverter). I didn't think of trying this originally because I figured it would make the swings worse, but it made them better. After experimenting with different settings of width and offset, I settled on a .7 offset and 3.0 width; this produced a much more manageable 2 volt swing on the AC side. I basically tuned it by ear to get the steadiest hum from the inverter.
So for whatever reason, I guess the Magnum inverter likes the 'slower' frequency of the wider pwm width.
The minor drawback is that this setting is not maximizing the draw from the panels, but I'm still getting plenty of hot water, so I'm not going to fuss over it.
Incidentally, I'm still using the 4KW element on my heater since I appear to be stuck with it; literally. My heater is 21 years old and no amount of cranking with a 6 point socket and breaker bar would budge it. As it happens, the pwm setting is not drawing more than 2kW even under full sun, so I'm inclined to leave the element alone until it fails.