Welcome to the forum.
I believe National RV used Velvac as OEM. What year is your Tropi-cal?
It it's a 2004 to 2008, the mirror is a Velvac 2030 (see attached page below) If you go to the Velvac website, you can order just the parts you need or the entire assembly ...go this THIS PAGE
The whole thing appears to be $527.13 (part #714823).
If it's an earlier Tropi-cal, go to Velvac's reference page for the Tropi-cal:
Velvac Class A Reference
So yes, it appears they are charging you too much as you can order the part directly from Velvac and then put it on yourself or hire a mobile RV repair person to install it.