Thanks to everyone who replied for your advice and information. There are obviously lots of pros and cons between the two types of batteries. We do spend a lot of time off the grid so that is one factor not present for many RVers. But, when I started shopping I was under the misimpression that AGMs were lighter, obviously not so.
My first thinking was to buy two Interstate 6V golf cart batteries, having had good experience with those in the past, and, after digesting all the info posted here, I'm again leaning in that direction. At about $150 apiece the difference in price isn't half that of AGMs, but significant. If anyone can suggest a lower price alternative, equivalent in quality to the Interstates, please do so. I have the idea, possibly wrong, that they should be above 200 Ah (20) rated.
I don't even have possession of the new TT yet so still have some time before making a final decision.
Thanks again to all of you, I really do appreciate this.
Mike