I have mixed and matched various combos and it does work out ok while camping. The high draws are just for a short time and as you say, the low draws are not much of an issue. In between the batteries "middle" as best they can. Actually that is a slow process with its tiny voltage difference between batts, the AH exchange is glacial.
With the extra batts, even when dissimilar, you do get way more AH and need to recharge less frequently, which is the object of course. Also the inverter does not hit its 11v alarm so soon with the extra AH on the job.
It does not matter if on a recharge one gets higher than the other because after you stop at imagined 90% they will middle sort of and start a new cycle downwards. Nobody is measuring exact numbers.
As mentioned it really matters after you get home and need to get each batt to 100% true. Got to separate them for that.
