"We have a 20 amp charger for our gel batteries on board and with a generator, we only get 10 amps/hour"
In the simplest case, what is wrong is that your generator can only supply thta much power to the charger, so what you will need is a larger generator.
But it could be that, or a defective charger, or the batteries not being able to suck up the larger charge, a number of things. Don't assume randomly throwing a more expensive part into the system will fix anything.
You could document and test the entire system, with some reading and some help online, or you could pay a marine
electrician to check it out. But randomly buying
"more stuff" ? Is often the slowest and most expensive way to go.