I have been using SU 2017 for years and have just now had to upgrade to SU 2018 due to the SU warehouse removing old SU 17 downloads. Since I have made the switch I can’t find any models for 2018 either, so I export as a Collada file based on other’s advice.
The problem I’m having now is when I import a Collada file to sketch up 18, all linework is visible. All are still displayed even with hidden geometry off. I have no idea what’s going on with this version. This is why I try to stick with a single version, but I must learn newer now.