Looking at the one rated for 3000 watts, which would allow for the A/C and one major appliance to run, has that tremendous price tag. I wouldn't be looking to spend that much soon after driving off with a new trailer.
Something I came across last night talked of having the converter, inverter, and transfer switch each seperate as if there is a problem it's much cheaper to replace, not to mention portions still work not leaving you in quite as bad of a position.
I saw what appeared to be a good (3 stage and sine wave) 3000 watt inverter for under $500.
How does one know if the converter that comes with the trailer is of a better quality? From what I read in several different articles/blogs is that it's common to get a poor quality one on older trailers, but that some nicer trailers may come with a better one. From what I gather the Keystone Passports aren't quite top of the line, but are maybe 3/4 up the ladder.