"I'd say look into the env_reset and env_keep options in man sudo. But it sounds like you've already done that (you just mistakenly call env_keep ""keepenv""). If you disable the env_reset option (default is enabled), I think it's not supposed to erase any env variables. But this is less secure.
There's also a secure_path option to sudo; I think this is enabled by default. You could try disabling it.
The preceding options are set in your /etc/sudoers file. There's also the -i command-line option to sudo. That will cause sudo to run /root/.profile or /root/.login. You could set your desired path there."