Go to some place like PPLmotorhomes dot com, or Camping World dot com rvupgradestore dot com.. pick one that fits the hole, they use a "trim kit" to cover the excess..
The model your 177 probably would have come with would be a Sunbeam, but over the years FR has used several different brands and models.. all the 2011's I have looked at or worked on had the Sunbeam, but it hasn't been available for years.
As a note.. I see higher end RV's now coming with microwaves that are "platerless".. I like this idea very much, as keeping track of the glass platter and it's hardware while traveling can be a problem. I do not know if there is a convection model with this technology yet or not.