It's a misnomer that passenger vehicle tires cannot be used for trailers. In many ways they are superior to trailer tires - PLENTY (I mean tens or hundreds of THOUSANDS) of people in the RV crowd have switched to using LT (light Truck) tires on their travel trailers. I had LT's on my last 5'th wheel travel trailer. Trailers have travelled millions of miles on them and the results are almost universally better. Go read over at rv.net if you want plenty of proof.
The problem with ST trailer tires is that increasingly, almost all of them are JUNK. They are made in China and the quality isn't there. They come with the assumption that a lot of trailers travel only a few miles every year - a lot of RV'ers rarely tow more than hour from home. They are built to within a hair of their ratings. There are often quality issues that don't appear until they fail.
They are also notorious for blowing up. You only need to search the term "chinese ST trailer tire bomb"
for some very enlightening reading. Spend some time over at rv.net and you'll soon read about all the RV'ers who went to the LT tires years ago and have never looked back.
You CAN buy good quality ST tires, but they cost about the same as LT's, are harder to find (vs easy to find LT's), and aren't that much different in the end. In the end, LT's are the preferred option for many now. The only thing you need to make sure is that they are rated properly - you may have to buy a higher rating (higher ply count) LT tire to achieve a proper weight carrying capacity - this is essential, and really, no different vs buying an ST tire.
As for having a non-trailer-savvy mechanic looking at the trailer, it shouldn't be a problem - trailers are simple. Reiterate that you are most concerned about structural integrity and safety with him and that you can't (or won't) take any risks if ANYTHING doesn't look right. It's not rocket science and anyone that can identify a bearing from a light bulb should be able to identify any problem areas.