I am far from an expert, but here's my opinion...The "top off" stage of battery charging happens very slowly. I feel you would get more towards your goal of cutting your generator run time by spending your money on another pair of batteries. This would allow you to build up to your needed charge quicker by only utilizing the "bulk" mode during generator use. You would "top them off" when hooked to shore power.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)