The following excerpt from the link below and associated RFC2401 does indicate TOS values are preserved when IPSEC tunnel mode is used.
http://www.cisco.com/warp/public/105/crypto_qos.html#t4
Cisco IOS Software Release 11.3T introduced support for IPSec and along with it the ToS byte preservation feature. With this feature, the router automatically copies the ToS header value from the original IP packet to the encapsulating IP header when the IPSec in tunnel mode is used.
Cisco PIX Firewall version 5.1 and lat