I don't use spanish letters myself, but during the years I have used
xmodmap with a swedish keyboard.
Defining the Multi_key in itself will not be enough to get special
characters. You will also have to use xmodmap to tell how some key should
be modified with the Multi_key.
The command
xmodmap -pke | grep Multi_
gives the following output on my system:
keycode 134 = Multi_key Multi_key Multi_key Multi_key
and the command
xmodmap -pke | grep -i quest
gives the following output on my system:
keycode 20 = plus question plus question backslash questiondown backslash
My guess is that the output from the latest command on your working 14.2
system is what you want to achive on your 15.0 system.
regards Henrik