As Granite said SP with mods is tedious with all the issues it comes with but definitely not boring. For me I find modded SP more immersive and you get to actually experience the feeling of trucking, and there is still stuff happening in front of you all the time. In TMP though unless you're in the busy areas driving on an empty highway at 110km/h can be a bit meh.
I used to always play on TMP, but when I decided to give SP a shot I found it to be boring as well but after sticking with it a bit, watching people playing SP with the crispy graphic, sound mods and modded trucks (way before the 1.40 update which introduced the new lighting, oh and before save editing as well) I managed to fall in love with it.