Perhaps I wasn't clear. Paralleling lead-acid batteries is done, but
its not a good idea. If series is possible then use series. I believe
your reports of using them all over diesels where there is no other
option that cheap, but that doesn't mean it is a good idea for a little
trolling motor.

Shortwave Sportfishing writes:

"Failure mechanisms of deep cycle batteries are described on pages
14-97 thru 14-98 of reference 1. Grid corrosion and plate shedding are
the predominant failures. Neither is a result of parallel
connections."

A thoroughly shallow analysis, followed by ignorant opinions like: "a
cell rarely, if ever, shorts with a low resistance". All from somebody
trying to sell you something.

I suppose if you have never seen a car battery explode, you might not
believe the warnings, or at least you might not treat them with respect.
I know I didn't, until it happened. I still jump car batteries when the
need arises, but believe me, I do it with care and wearing protective
gear.

I'm not gonna get in a citation war. Look at lead-acid battery mfr
sites and you'll find they typically caution against paralleling if they
address the issue at all.

This religious advocacy of a poor practice reminds me of people who
advocate using suicide cords for portable generators. Yes, they work,
yes its done (and I have done it myself), but it isn't a good idea, and
it risks some serious problems.