What are the limits to connecting batteries in parallel?
The risks increase with charge rate as current mismatch  is dependent on matched cells capacity and ESR. 

So if one battery  is weak (low C(Ah) or high ESR from sulphation) then the others   carry the load.

If you have a common ground and heavy wire to each battery (-) all the same length, you can monitor the mV drop using the cable as a current shunt. using a rotary switch and DMM using the shared Gnd for Vdc in(-) on DMM.