UPDATE:
I found 6v golf cart/RV/Solar batteries at Costco here in Canada. $139 each (plus a $10 eco-deposit) for 208 Ah units. I also received the IQ4 from Amazon.ca, it was $35 shipped. Not bad!
So, now as soon as all this snow is gone and the temperatures get above freezing, I will get the trailer out of storage and install my new goodies.
As a side note, I will not be parking the trailer in front of my house with the batteries on the tongue, they've become a popular item for thieves. They get $10 each at the local recycling yards. They usually are in a stolen pickup or SUV, and spend the night stealing. They use bolt cutters, cut the battery cables and off they go.