The file is written correctly. It is just that the function load() can't deal with it. If you read the help of load(). It says "With -ASCII, LOAD will error if the file is not numeric text.".
You can use importdata('mongiss.tv') to read the data. The data will be read in a structure though.
But to verify that you can append characters to a text file with numerical data, you've achieved the goal. You can use a text editor to verify it.
