Haven owns and operates 35 holiday parks in the UK, and is part of Bourne Leisure Group.
Bourne also owns other UK brands including Butlins and adults-only hotel specialist Warner Leisure Breaks.
This summer, reports in the marketing press said that Warner – which only advertises online – was looking for an agency to handle its digital account worth £1.5 million.
Haven's move to relaunch its website follows a £41 million refurbishment of its parks, announced at the start of the year. The new site is scheduled to go live in November.
The deal is the latest win for Nucleus, whose other clients in the travel sector include Teletext, Ocean Village and Transport for London.
Financial terms for the Haven project were not disclosed.