With only 15U, assuming devices don’t stick out the back, I’d move it face-up, so devices are more hanging from their ears than cantilevered. A full, 42/48U rack is extremely top-heavy and tipping during move is a serious risk, but 15U is fine. It’s still very dense, and OP should try to ratchet-strap it to hard points in the trailer.
It really depends on what your data is and how hard it would be to recreate. I keep a spare HD in a $40/year bank box & rotate it every 3 months. Most of the content is media - pictures, movies, music. Financial records would be annoying to recreate, but if there’s a big enough disaster to force me to go to the off-site backups, I think that’ll be the least of my troubles. Some data logging has a replica database on a VPS.
My upload speed is terrible, so I don’t want to put a media library in the cloud. If I did any important daily content creation, I’d probably keep that mirrored offsite with rsync, but I feel like the spirit of an offsite backup is offline and asynchronous, so things like ransomware don’t destroy your backups, too.