Officers from Hertfordshire Constabulary carried out the operation in Mallard View, Billing Aquadrome at about 6am today (Friday).
Neighbours said at least 10 officers descended on the caravan and the pair were taken into custody.
A spokeswoman for Hertfordshire Constabulary said: "This is in relation to an incident that occurred in Hertfordshire in June 2018.
"A man and a woman from the Billing area were arrested in relation to drugs and firearms offences."
The raid saw detectives, dog handlers and firearms and forensic teams take part.
Bomb disposal experts were also sent to the scene over fears that items found inside could be dangerous.
However, the spokeswoman said: "The EOD (Explosive Ordnance Disposal) team were deployed to identify some items discovered during the warrant. These were found to be non-suspicious."