I just installed R3.5 (and RStudio) on a new computer that has not previously had R on it. I then installed the devtools package, and when I went to install a package off of github, I realized (when prompted!) I had forgotten to install Rtools, so I followed the prompts to do so. Interestingly, this R tools installed to the directory of C:\RBuildTools by default and with no prompts for setting path variables as I had recalled in previous versions.
When I reloaded R, I was prompted to install Rtools again. My path contained no reference to Rtools (or the Rbuildtools directory), and remembering that my past installations had used the dir C:\Rtools, I decided to uninstall C:\RBuildTools and installed a fresh version off http://cran.r-project.org/bin/windows/Rtools/ ( installed version 3.5) into C:\Rtools. In installation I had the path variable option checked.
I reloaded RStudio and checked to make sure Rtools was found:
> devtools::find_rtools()
Error in system(full, intern = TRUE, ignore.stderr = quiet, ...) : running command '"C:/PROGRA~1/R/R-35~1.0/bin/x64/R" --no-site-file --no-environ --no-save --no-restore --quiet CMD config CC' had status 2
However, rtools appears in my PATH:
> Sys.getenv("PATH")
[1] "C:\\Program Files\\R\\R-3.5.0\\bin\\x64;C:\\Rtools\\bin;..."
So this doesn't appear to be an issue with Rtools not being added properly to my Path variable. Google searches have turned up seemingly related errors but nothing so far that solves my issue. I have tried both suggestions here and neither have worked. I wonder if this could be related to RStudio.