But is very easy to add support for RMVB files in totem, you just need to install w32codec pack.
For a standard Ubuntu system (maybe work for other debian base distributions) you just have to do that in a console:
1- Download the w32codec pack:
wget -c http://packages.medibuntu.org/pool (Note: this is just one command)
2 - After download the package install it:
sudo dpkg -i w32codecs_20071007-0medibuntu1_i386.deb
Now you can view RMVB files in your Totem media player Enjoy !