I would certainly want to see insulation tests on the installation and appliances first to rule out anything obvious.
Yes although my installation tester uses DC, and a capacitor will not pass DC but it will pass AC, so a filter can allow a small current to earth with AC, also the cables with have some capacitance and inductive linking, so where you can't hold a RCD in then clearly you start with an insulation tester, but where the RCD is holding then you need some other method to see how close to the tripping current you are.
Most 30 mA RCD's trip around the 25 mA mark, they are required to trip between 15 and 30 mA and most RCD testers have a ½ setting, so check it does not trip at 15 mA, if the test is done with all connected then one is reasonable sure the back ground leakage is not on the edge, but for the 30 mA test if there is a leakage of 9 mA it means it may need 39 mA to trip, from bitter experience I know strain from the connecting wires can change the sensitivity of a RCD, so it needs testing with all cables in place and torqued up.
So only real way is to measure the back ground leakage, however in real life RCD's tend to trip before anyone touches anything live, when my roof leaked the RCD feeding the sockets under the roof tripped well before anyone touched them, so in real life it does not really matter that much if they don't trip bang on 30 mA. What is more of a problem is to ensure they don't trip with back ground leakage.
We all do it, we walk into the house, the RCD has tripped, we reset it, and only after resetting do we consider food in the freezer, with my upright freezers it displays the highest temperature it reached before power was restored until you open the door, but not all do this, and how warm a freezer gets depends where it was on the defrost cycle and how much is in the freezer, so if only just finished defrost cycle when power goes off, the time is very short maybe ½ hour, but if just switched off freezing, it may take 12 hours to defrost.
I am no biologist or cook, I don't know the dangers of refreezing food, so all I can do it reduce the risk of power cuts to freezers to as low as I can. The same applies to trip hazards with power cuts, I want to make it so unlikely to loose lights, use of auto lighting rechargeable torches, and the lights do not share a RCD with anything else. Maybe not always best idea as I can loose lights but still have sockets so the torch will not auto light, but chance is slim.
But no one is telling the home owner what to do, he needs to make his own risk assessment, if the risk of having spoilt food is low, he may consider one RCD is enough, I lived in a caravan with one RCD for all without a problem, OK gas fridge, no freezer, and battery backed lights, but it is up to the owner to do the risk assessment. No one is forcing you to have multi RCD's, but if a single RCD trips it you paying the bill.