best way to test house batteries ?

I test with them separate. Then check each with the hydrometer after they set for an hour or so. I also check each cell with the volt meter after the hydrometer test. Between the hydrometer and volt reading of each cell you will find any weak or dead cells. Your lights will not put a load on the system. Load meters are $25 or so. I only use that if I am having a issues on the weak cells.

If you battery charges up good but will not hold voltage after 4 hours then you have a weak/dead cell. Should drop close to 2+ volts for a weak cell. A weak cell battery will pull any battery that it is connected to down.