It was low battery. Barely could start genny.
I guess my battery tender wasn't keeping the batteries charged. Its a Battery Tender Plus. Perhaps it thinks the batteries are charged when in reality, because they are deep cycle batteries, they run way down before the tender thinks they need to be charged?
I've noticed if I disconnect it and reconnect it, it will charge for days before it thinks they're fully charged. The trailer has been disconnected for a couple weeks on top of the tender not being in charge mode for a while.