Your man's death while in service would have been noted on muster- and payroll if these records survive.
The National Parks Service's website listing personnel at Valley Forge 1777-1778 lists only one Lewis Campbell, a Pvt. in the 4th NJ Regiment (Continental). He is not noted as having died at Valley Forge, though he certainly may have died subsequently in the same year.http://valleyforgemusterroll.org/
--click on the "Muster Roll" link at upper left to get to the search box.
You can look at the surviving rolls for the Regiment your man was in using a free trial period on ancestry.com or search the Compiled Service Records on www.footnote.com
(which also has free trial periods). Your local library may subscribe to either or both of these services for patron use at the library.
If you know what Regiment your man was in you can ask your local library to borrow its rosters on microfilm for you to look at. This site lists the surviving Continental units by what microfilm roll number they are in within National Arcives Micropublication M246, "Revolutionary War Rolls":http://www.revwar75.com/ob/m246.htm
A burial site for a Lewis Campbell has not been published in either the NSSAR or the NSDAR Patriot Graves publications. Local chapters near where your man's family lived during the War might have made some memorial, however, that was not included in National Society publications.
While descendants may have had his remains reinterred in a National Cemetery, the Veterans Administration does not list him.