Trailers and caravans do not need a separate e-vignette in order to use Czech motorways, only the towing vehicle up to 3.5 tonnes requires the e-vignette.
Vehicles with a maximum permissible weight of over 3.5 tonnes (incl. combo of vehicle + trailer) use a distance-based toll system (MYTO CZ) instead of the e-vignette.
Related articles:
What is the MYTO distance based toll?
Where can I register for the MYTO CZ and get the on-board unit (OBU)?
What vehicles need a Czech e-vignette?