Approximately what percentage of aircraft are equipped with RNAV?
Use of RNAV began in late 1960’s . Some airlines began exploring RNAV systems  


KLM/SwissAir/SAS/UTA had a cooperative effort
Swissair became responsible for the development of a database to
support this effort


In June 1973, National Air DC-10 equipped with Collins ANS-70 conducted RNAV operation, including approaches in VMC, with database. 

In January 1983, the Federal Aviation Administration revoked all RNAV routes in the contiguous United States due to findings that aircraft were using inertial navigation systems rather than the ground-based beacons, and so cost-benefit analysis was not in favor of maintaining the RNAV routes system.

RNAV was reintroduced after the large-scale introduction of satellite navigation.And, since 1990 it is there in almost all the planes you see.
So, as a rough estimate it would be more than 90%. Because, most of the pre-1990 planes have either been upgraded or grounded.   

Sources: Wikipedia, Federal Register, This Article.