I would recommend one of these:
https://landairsea.com/products/silvercloud-sync
I have them and they work well. I particularly like the GeoFence feature. There is a monthly charge for the tracking service but to me it's cheap insurance.
These units have an internal battery so they will continue to send signals even when the power is removed. There are reviews that state some customers charge them at night and then throw them in their kids backpacks before sending their kids to school. They can be plugged directly into an OBD port. They also sell a power cord so they can be placed anywhere. Using a power cord lets you place it under the hood instead of under the dash. In a trailer that always has battery power, you could hide it just about anywhere. Same in a boat. If your trailer does not always have battery power, you could hook it to just about any trailer wire circuit to make sure it went off.