An update....
I took both T105's out of the RV this morning. One is on the Shumacher 6V 6A charger, the other on the NOCO Genius.
Shumacher showed about 4 amps of charge-rate at the beginning (about 2 hours ago) and has since fallen to below 2A.
SG reading is 1275 on all cells, so we seem to be making progress....cautiously optimistic so far.
I haven't taken any readings on the NOCO battery as it has a maximum 2A charge rate and is currently in "maintaining mode" with the green LED pulsing.
I will probably leave that one until this evening at the earliest.
As I overcome one problem, I start hypothesising about others....maybe I had "enough power" last year because the batteries were so new and we definitely had more sunny days than this year...it has been a dismal Spring and unimpressive Summer so far, in terms of hours of sunshine.