If you do a load test you will need 24-48 hours to charge the battery back to 100%.

"monitor showed the house bank at 82% charge and 12.1v at-rest"
12.1v I would think that would be 50% at least less than 70%.

"Volvo-Penta (Valeo Rhone) 60a" "Soon after starting underway I was only getting about 15-16 amp"
I would think with a 30 Amp alternator 15-16 would not be bad. They cut back fast from their rated output. Not sure what a 60 amp would do.

"after about 2 hours I checked again and it showed about 13.8v and acceptance at around 7amps"
With only 7 amp out I would want to see 14.2 I think your alternator is still set up for gel cell battery or you have a battery with bad cell. This is telling you something importance I am not sure what. At 13.8v all battery should be tied together. I would see if I have 13.8 at each battery. I would check each battery cell with a hydrometer to see if one cell does not match the other in that battery.

"at around 7 amps charge current, and about 87% charged"
At 87%, 7 amp seem maybe a little low, I would think around 10 amp.

"house bank at 82% charge and 12.1v"
That can not be a true statement. (I believe that is the reading) 2 hours a 15 amps is not enough to bring 3 battery up to 85-90% where I would think 7 amps would be correct. Or are you saying one battery is reading 12.1 and the monitor knows there are 2 more battery so it is saying 87%.
You start the engine and the alt is charging one battery 15 amp would seem right. Then at 2 hour the voltage get above 13.2 and all battery are tie together. At this point 13.8 maybe OK but 7 amp seem way low.

I would check the volt at all battery when charging above 13.3 and do the hydrometer on each cell for more information.