An incident room has been set up by Humberside Police
Stanley Twyman, 61, died in hospital after being found with serious injuries - alongside another watchman who had also been hurt - at Atlas Caravans on the Dairycoates estate.
The pair were taken to the Hull Royal Infirmary after the alarm was raised in the early hours of Tuesday.
It is believed that both men had suffered a severe beating from the three raiders.
Former serviceman
Police said it was not known at this stage whether a weapon was used in the attack.
Mr Twyman, 61, from Bransholme, Hull, was working for Hull-based firm SPS Security.
The father-of-three was a former RAF serviceman who had been awarded medals for long service and good conduct.
Family 'devastated'
In a statement released through the police, Mr Twyman's family paid tribute to him as "a very private man who loved his family".
It said: "He was a normal man and a very good man who had never been in any bother in his life.
"Today (19 September) will be the third birthday of one of his grandsons and Stanley had bought him a bicycle.
"We are devastated that he cannot give it to him."
A police spokeswoman said that Mr Twyman's colleague, a 62-year-old man from Hull, had been released from hospital after treatment.
Detective Superintendent Quentin Dowse, who is leading the investigation, said: "These two security guards were just doing their job.
"The men who carried out these attacks obviously gave no regard to these men or to their families."