Same thing happened to us last year. I tightened the wing nuts on the battery terminals. That worked for a few days then happened again. Did this a couple times and the last time it didn't help.
Two batteries bad. Swapped out both for 2 new Group 27 deep cycle and everything was fine, no dimming of lights , pump running fine , and the wingnuts stayed tight on the new batteries for the remaining 2 months of the trip. Didn't have to tighten them again.
Check the battery connections, if that doesn't help, pull the battery out, take it to Auto Zone or O'Reilly's or a battery shop and have the battery load tested. It is a free service, unless you have to buy new batteries.
