The Trailer Stories for Girls series was written under Millie's own name of Mildred A. Wirt. Published by Cupples & Leon, this series consisted of four titles:

• volume#1 The Runaway Caravan, 1937
• volume#2 The Crimson Cruiser, 1937
• volume#3 Timbered Treasure, 1937
• volume#4 The Phantom Trailer 1938

Trailer Stories books are hard to find, especially with a dust jacket. Millie promoted other books she had written in Runaway Caravan. In this volume, one of the main characters, Barabara, has books in her room including some of her favorite stories-- The Twin Ring Mystery, The Clue at Crooked Lane, and The Hollow Wall Mystery, which she has read so many times that she can repeat parts of these stories from memory. Twin Ring is a volume in a Mystery Stories for Girls series by the same publisher of the Trailer Stories series and Crooked Lane and Hollow Wall are part of the Mildred A. Wirt Mystery Stories series, also by the same publisher. It appears that the first three volumes were issued as a "breeder set" in 1937 and then only one more was published the following year in 1938.

The Trailer Adventures:

The Runaway Caravan:

Touring the country in a second-hand trailer, Barbara, Ginger, and Jimmy run headlong into a series of adventures. Before they reach home again, they discover some amazing secrets about their trailer.

The Crimson Cruiser:

A bright new crimson trailer takes Barbara, Ginger, and Jimmy to Great Falls in Arizona where they become involved in a maze of events, including a strange Thunderbird, the mystery of the Valley of Voices, and an abandoned mine.

Timbered Treasure:

Adventure and the lure of the open road take the three youngsters to Maine's huge timberland where they help the Forest Rangers solve the mystery of forest fires that occur simultaneously with great thefts of timber.

The Phantom Trailer:

A dark ghostly trailer moving across a deep gorge of rocks to disappear among chalk white cliffs in majestic Yellowstone Park, a notorious bond thief, an elderly rich woman with her grasping nephew--these are the elements of mystery and adventure that challenge Barbara, Ginger, and Jimmy.