Dan, I can't speak specifically about the Excella, but generally speaking, you don't want to draw more than what the original system was designed for. If the Univolt converter is a 30 amp, then you want to make sure that the new converter doesn't provide more than that on the existing fuses and wiring. That's for the DC voltage portion.
Most of the items you mention are going to be powered off 120 vac, so you want to make sure you don't exceed the maximum rating of the main circuit breaker. Again, probably a 30 amp breaker.
There are ways to split up the loads through a 4-6 breaker panel for the 120 vac, and you can also add circuits to the DC portion and change-out the old fuse panel, but it gets kind of technical.
I think most people replace the old Univolt with a new electronic converter/battery charger because they're more reliable and are better for the batteries. I switched to an Intellipower, and it seems to work fine.
Keep in mind, the plug connector for shore power is probably only rated for 30 amps, or maybe 50 amps, vac.
If you search through the forums, there's quite a bit of discussion on the subject.