Error Code P0300 '05 Frontier

So my truck is not working well at all. Very reduced power so it just limps along. Getting this error code and dealership said to replace spark plugs. Did that and then they said replace coil packs. They wanted to charge too much so I got some aftermarket ones and did it myself. It still has the same problem and same error code. Took it back to dealership and they say they think the coil packs I bought are not good and that I need to use theirs.

I don't see how that's possible though. If the ones I bought weren't good, then how is the truck running exactly as it was before I swapped them out? Wouldn't it now be not running at all if they were faulty? They are convinced it's the coil packs though so I suppose I will have to buy theirs, get them installed and see what happens because they just seem to refuse to look at other possibilities until we have ruled this out. Any advice would be helpful.