well, you're required to have an operational battery while transporting the trailer. this is because the trailer's emergency breakaway switch needs 12v power to operated the electric brakes, in case the trailer detaches from the tow vehicle.
i had 2 12v deep cycle batteries from Costco, on my hybrid. they lasted 9 years and were still working when i sold it.
the prices you're quoting are too expensive. try Sam's or Costco.
if you never plan on dry camping or boondocking and will always have shore power, a cheap 12v group 24 marine battery will be fine.
