We purchased a multi function printer/copier for the boat that had a 120 adaptor that outputted 12V. I cut the cord and changed the plug to one of the cigarette lighter plugs we had all over the boat. The only time I didn't use the printer was when we were equalizing. Worked fine and we didn't need to use the inverter. 12.8V does seem a little low on the voltage tolerance. Most stuff that operates on 12 volts will take a higher voltage than that without any problems. Could be listed just to cover themselves and also to keep you from doing what you're thinking of. Than again maybe not. Our printer cost about 125.00 and I was ready to accept throwing it away if I screwed up. Not using the inverter was worth the risk.

One thought. Most 12 volt outlets don't have near the same voltage as is displayed on your output panel. In our rear stateroom the outlet voltage was almost 1/2 volt less than the panel reading. Of course, your milage may vary.

Personally, I'd probably do it.

I have a saying, "I don't know what I'm doing quite often but I do that a lot so I'm pretty good at it".