Easy way is to get the specifications for the on-board charger. If it is a three-stage unit with defined voltages during absorption and float stages and you trust the unit to be working, then you can assume it is doing its job. Or you can get a digital multimeter and read the voltages during a whole charging cycle and if they match what the specs state, you should be OK. As a final test, once the float stage is reached, disable the charger and fully disconnect the battery(s) and leave it sitting for at least 4 hours and then measure the voltage. Anything over 12.6 is a pretty good indication that they were 100% charged.
Also, need to check the electrolyte level every week until you get a feel for what is happening. Needing top-up more often than say two-monthly (but it varies from rig to rig) MIGHT be an indication that the charger is hitting the batteries harder than necessary and maybe the cycle settings need modifying.
Because every rig has an unique set of operating conditions so it is hard to give general guidelines covering all situations, but even though the resting voltage method has its inaccuracies, it is still a pretty good guide to actual state of charge.
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper