If the 31 is anything like the 1275, you don't need to get it full every day. You can just use it however for some number of days and then goose it up and start over.
I don't get the voltage drop under load issue. As long as it stays above inverter alarm under whatever load, who cares what it actually drops to as long as it bounces back?
IMO now that it is known that it will come back to baseline after not being fully charged for a while (as before when first installed) just ignore the thing except once in a while do the 100% on it. Don't let the battery drive you crazy :)