This topic is addressed in other threads. I reiterate that the correct answer as to just how fast one should charge deep-discharge lead-acid batteries (designed for cruising applications which infers sufficiently large cell interconnects as opposed to the small ones used for example in emergency lighting
systems in buildings) is expressed by the Amp-hour law. This "law" states that a lead-acid battery can be charged at a current
equal to the quantity of Amp-hours depleated from the battery without excessive gassing or temperature.
This "law" comes from one of the "ruling" textbooks on battery electrochemistry. The reason that you have not seen this information disseminated widely is because no one has commercial equipment
available to perform this automatically. First of all, it takes a proper battery monitor
to be the basis for the control. Again, gelled-cells and absorbed-glass-mat lead-acid batteries fall under the same category in this regard.
I have done this charging many many times to all of these types of batteries with wonderful success and, in many cases, have brought all of these types back from the dead when they were discarded by others who thought that they were a lost
BTW, the value 15.5 V for a charging voltage is not a good value to use. Just as the Amp-hour law implies that the safe charging current is ever-varying the correct compliance voltage is also. I have noticed, for example, that in the electrolyte starting temperature in the range of 65 to 75 deg. F 15.5V would cause excessive current in a battery in good condition and exhibiting good charge acceptance (although 15.5V COULD be applied for short periods without damage, again here we are talking about a charge regimen, not a short deviation which all these batteries will tolerate for awhile without damage).
Voltages in the range of 15.5 V to almost 17 V may be applied to fully charged batteries in order to equalize them using constant-current sources (the proper manner to equalize batteries). Now no one that you know has such sources, I'll guess, and we all know that one can approximate giving a proper equalization
charge using voltage soucres (like modes offered by inverter/chargers) and limiting the charge voltage to 15.5 Volts instead of allowing wild currents to flow with higher voltages not having current limit capability.
The reasons that "the industry" has such disparate recommendations is manifold: the sales outlets must limit their liability for warranty expenditures, most users do not have proper battery monitors, most users do not have proper charging sources, very little rigerous technical information resides within the industry outside of the electrochemists themselves residing within halls of the big three manufacturers and that which is promulgated is done so as "rules of thumb" not based in rigorous science. In addition, there is a great wealth of myth within the cruising community regarding many aspects of technology having only some basis in fact which makes such information difficult to dispell.