Hi
I don't know much about the Victualler's in Watford from that time frame but I'm sure there were plenty of Inns, I have at least 3 ancestors who were licencees in the early 1800's in Watford & the surrounding area.

From Family Search records there does appear to be a John Dawes in Watford, possibly 2 unless it was a second marriage...... although I can't find any marriages at the mo, however, John Dawes & Elizabeth had 2 children baptised at St Marys Watford, Mary 30 Sept 1746 & Elizabeth 20th May 1750.
John Dawes & Mary had 4 children baptised at St Marys, William 15th Feb 1756, Richard 18 Dec 1757, Thomas 2nd Dec 1759 & Ann 19 Dec 1762.
There is also a Joseph Dawes & Mary who have a Thomas baptised at St Marys 27th Mar 1750.
Looks a good probability that this is the John mentioned in your Williams Will.
Hope this is some help.
Maddie