Green’s Dictionary of Slang

1. a helper.

1901		C.R. Wooldridge Hands Up! 44: A man or boy [is] stationed in front of the houses, who is called a trailer.
1928		M.C. Sharpe Chicago May (1929) 56: I slipped my money to the trailer (helper) and returned.

2. (US) a companion.

1911

E.B. Morris Sophomore 27: Stewart. Have your aunt and her trailer — Hope. Now, don’t be disrespectful.

3. (US tramp) a tramp who follows the circus.

1927		in ‘Jargon of the Und.’ in DN V.
1931		Irwin Amer. Tramp and Und. Sl. 190: Trailers.– Tramps or beggars or ‘grafters’ who follow a circus or carnival.
1949		Monteleone Criminal Sl. (rev. edn).