If you're adding batteries to existing storage, any mismatch is less than optimal for long cycle life. Best is when all batteries are matched. Even the same batteries are not matched when part of the array is aged. The good news is parallel connections are more tolerant for mismatch.
Since you have a new coach, your batteries "might" be ok. I say might because sales lots can be really hard on batteries and age them early. Assuming your factory batteries are 95% or better, the best option for "adding" batteries is to get the same batteries to put in parallel.
Consider your interconnect cable lengths and how they affect your charge/discharge cycles. Matched batteries can become very unmatched due to long cable run losses.
One way to increase capacity is by going to a denser power chemistry and
drop in replacements are becoming more available. The LiFePo4 drop-in options provide a much better DOD and eliminates the importance of 3 stage charging. DOD is depth of discharge and that means you get to use more of the battery's capacity without degradation (think near 90% vs. 50% for FLA). Really the only downside is the initial sticker price! Long term however they could very well be cheaper YMMV.