On the US Thai visa application, theres a line that asks for your "Place to stay in Thailand"

How specific does this need to be? I don't haven't rented any hotels yet, so I just put in "Chiang Mai."

You could put any hotel name and address there. If asked you could say that is where you plan on staying.

I think it is a good idea to look up a hotel online and use that name. When you enter Thailand, they ask that question on the form they give you on the plane also. Not sure what they would say about just the City as to where you are staying, but I am sure many don't relly know where they will be staying when they first come to Thailand.

In case of non-working Internet on your phone when arriving at immigration, consulates, etc - I keep business-cards or do a lookup in advance, and put down the precise address/phone on immigration forms. This makes it appear you "have your stuff together," even if you plan to wing-it.

last time they asked me to write my cell phone number in case they need to call me . I think it s wise to write your email and phone number.