Zamp vs Battery Tender

Grumpy_G

Technically you can do that but it won't really charge the battery, just add some more capacity. That's because to charge you need a higher voltage than the external battery would provide. You could just plug the trailer into a Jackery or similar and use the converter to actually charge. Obviously leave everything switched to DC or gas and only use the AC power for the converter. 

pthomas745

This thread has a whole rundown on rigging up extra batteries (spares).  One has a Bluetti plugged into a port built inside the trailer.  Others just go right through the 7 pin plug with an easy adapter.

https://tab-rv.vanillacommunity.com/discussion/7008/anyone-carry-a-spare-battery/p1