Circuit Charging Sealed Lead-Acid Battery (NP1270M)
That's a nice datasheet. Trying to work out what the curves mean would be educational - but it's not essential for your purpose.

In your application you are essentially "floating" the battery.
The other typical use is deep discharge.
Your application lies somewhere between but closer to floating.  

13.5 to 13.8V maximum voltage when charging (float charge) is good enough for your purpose.
Charge at not more than 1.75A and ideally at around 0.7A until this voltage is reached. 

Occasional increase to 13.8-14.1 (Equalise or boost) will help maintain capacity, but is not essential. 

Do NOT float at 14V plus - that is only for recharge after deep discharge.