I can't say I've observed that trend. I haven't done any scientific studies or anything like that. I have owned exactly two "new" RVs. One Motorhome and one travel trailer. I've also owned other motorhomes. I can't say I had significant warranty issues with the RV portion of either. I had minor fit and finish issues in the beginning with both the trailer and MH. I haven't had any mechanical issues with the trailer, so far. I did have several mechanical/drive train issues with the MH while it was under warranty. Several recalls including verifying frame welds on the factory trailer hitch and replacing incorrectly installed catalytic converters. In addition to the recalls, had an intermittent engine failure that was finally tracked to bad wiring where it went through the firewall.
I tend to think coach related issues would be similar for trailers, FW, and MH from the same manufacturer in the same quality levels. I don't think it would be fair to compare the cheapest entry level MH to the most expensive FW or trailer. However, expensive doesn't mean reliable or trouble free either. Grand Design makes some very expensive and nice trailers, but they seem to have lots of issues.