Paul, as the situation stands. I am of the Opinion that you will have to play the long Game!
Once open & up & running. ENSURE You Keep a Log, & a visitors book ( Hard Evidence ) of the footfall of visitor numbers. The more you have come through the gate. Over a year or so, this WILL strengthen your argument for an established museum. & Worthy of Categorizing it as such. This with Promotional facilities, will spread the word. & bring in more visitors & money to the area.
When you are opening, Also ENSURE the press are informed. This will advertise & promote the Museum. & bring it to the attention of a much larger audience. 😉 local radio advertising it will also help! 👍