How bad is this?

At my first glance there is tape around the bottom of the mirror which means it was probably hit prior and is trying to put on you also no kind of debris on the ground that I see had you hit it. Also had you scrapped it I would assume it would be bent in unless it was moved back into position prior to pictures.

 

I'd report that you're being accused of hitting the mirror but you didn't. If you are unsure you still need to let your company know since the damage will be reported on his end.
Can you put your trlr next to the mirror and match damage?
If he didn't wake when hit and you don't acknowledge striking mirror it's becomes a "I said/he said" claim and you need to adamantly deny or your insurance will pay and you will get dinged.
Even if you did back in next to the damaged mirror but didn't touch it imho.
Looking close at pics he has taped it..
Tell him to go pound sand.

 

Where are the broken pieces from the 6 to 10 inches of missing plastic and the very thick mirror housing?

There is no debris.

It would be there, but where?

He swept and vacuumed the area up before he approached you?

There would be little pieces because plastic doesn’t just break clean on an impact.

what’s up with the tape?

the tape would have given way before the plastic. No scraping of the tape?

it looks like it was applied a day or two before.

You just don’t wake up, see your mirror was hit then tape it up immediately, then approach who you think hit it, do you?

Don’t these mirrors fold in on a heavy force used?

oh and also thinking about it, it looks like road debris hit.

I would report it to the company as someone trying to scam the company, send them the pictures. It is better to be upfront and honest than to look like you are hiding it, and I would tell them there was no debris anywhere and tape on the mirror which wasn’t damaged.

 

Ridgeline's reply above proves he is a mind reader, also.
You hit perfectly all the points I was going to go into.
The dead giveaway he's a scammer is the tape,
supported by the lack of small pieces of plastic.

 

At this point the insurance company notified first puts the other one on defense, quick, call - and say /I believe it is an insurance scam.

 

That mirror as 2 deep scratches on the bottom of the bracket where the arm is connected too.