An 8.8 is closer to a grade 5. Grade 10.9 is closer to an 8 imperial. I'd think 8.8 is fine for the trailer. It's not just the ultimate strength we're concerned about either, with the higher grade bolts, quality control is better, etc. You just want to get rid of all those 4.9 (=3) stuff. That's what you build decks with, not trailers.
I agree, for the price they're still worth it. After seeing all this, if I'd known they were for sale for $5000, I'd have bought one instead of building my own, and just deal with these issues. I've got $2000 in mine already, and that's just steel and suspension/tires. Don't even have a tent yet. Not to mention labor...