Thanks for the good comments. I will consider if Group 27 will fit. I'm a
little
confused by the deep-cycle vs. dual use. If deep-cycle will be OK for
starting
then why ever buy/make a dual-use?

You might compare the CCA and the scale weight of a dual purpose battery with
both a starting battery and a deep cycle. I haven't done so, but the comparison
might be interesting.

Starting batteries use thinner plates to create a greater area of exposure to
electrolytes. This results in a greater number of CCA in a given battery size.
Downside, those thin plates disintegrate faster during severe
discharge/recharge cycles..

Deep cycle batteries use heavier plates, at the expense of some CCA. The
heavier plates will withstand serious recharging with less damage than the
lighter plates in starting batteries.

Dual purpose could be anything. A starting battery with such a short warranty
or high margin that the mfg figures he can afford to replace a few under
warranty. A "medium duty" battery slightly heavier than a starting battery but
not as rugged as a deep cycle. A genuine deep cycle battery labeled "dual
purpose" to rake in a few sales from folks who think they have to have a
different battery technology for starting the engine than for running DC loads.
Who knows?