Humming and dim lights may be result of shorted cell in the battery due to over charging using the single mode 13.6VDC Maganetek converter/charger. They are known to do this over time. Need to check the battery fluids and if it is dry you probably should replace the battery. Once they boil out and short internally it is very rare they come back to life for you.
I haven't looked up the Magnatek unit but you may not have to do anything with the Power Distribution part of it. Just change out the converter/charger unit mounted in the bottom compartment. This will only require you to hookup the 120VAC side and two or three large battery cables... Mr Randy will tell you what units will fit in the bottom compartment of your unit.
I have the PD9260C smart mode converter/charger unit here charging up to four 12VDC batteries. After changing out my 45 Amp WFCO WF8945 that would never go into in smart mode charging I never any had more issues with the batteries. I still check the fluids however on a scheduled basis mostly out of habit.
I would go for the 45AMPS model or even the 55 amp model if you think you might ever add a second battery later on. Progressive Dynamics smart mode units are excellent converter/chargers with great track record. Each battery you have in your setup will need 17-20AMPS of DC Charge if you want to re-charge them to a 90% charge state in a quick three hour time frame. Almost all of the public places we go to here on the East side has generator run time restrictions in place so we need to charge up the batteries to their 90% charge state as quick as possible. Then we are good for the next day/night camping run off the batteries.
I got mine from AMAZON a saved a couple of dollars that BEST CONVERTERs was offering. Mr Randy is well known among the RV'ers... A great guy... Having said that don't let him talk you into a WFCO WF8900 series smart mode converter/charger... Almost everyone has problems with that brand going into smart mode charging when you need them to do it...
Progressive Dynamics or the IOTA models are very popular amoung the RV'ers...
Roy Ken