I checked the Wilhelmine's travel website, and the introductory text does mention a travel diary (without quoting from it), which is footnoted to:
Burrell, Mary: Thoughts for Enthusiasts at Bayreuth. Collected in memory of 1882 and 1883. [With plates, including portraits, and maps.]. Chapter IV. Unpublished Journal "Voyage d'Italie" and sixty unpublished letters of the Margravine of Bayreuth to Frederick the Great, together with sixteen unpublished letters from the King to the Margravine, London 1882.
So, over to you, native speaker, to Ms. Burrell's publication.
Re: Pöllnitz: Secret Keeper?
Burrell, Mary: Thoughts for Enthusiasts at Bayreuth. Collected in memory of 1882 and 1883. [With plates, including portraits, and maps.]. Chapter IV. Unpublished Journal "Voyage d'Italie" and sixty unpublished letters of the Margravine of Bayreuth to Frederick the Great, together with sixteen unpublished letters from the King to the Margravine, London 1882.
So, over to you, native speaker, to Ms. Burrell's publication.