As Brandon stated one of the biggest issues you’ll be suffering from is the reduction in available bandwidth as you traverse each repeater. If you haven’t already seen the Meraki mesh document,
https://documentation.meraki.com/MR/WiFi_Basics_and_Best_Practices/Wireless_Mesh_Networking then it’s worth a quick read. The more gateways (i.e. wired access points) you can establish the better.
It’s probably worth also understanding the absolute maximum throughput put you’re likely to achieve. The MR66 is an 802.11n access point with dual 2x2 radios, each supporting two spatial streams. Although the data sheet states up to 600 Mbps this is highly unrealistic. Although 802.11n supported 40Mhz channels on the 2.4GHz spectrum they’re barely used and the later standards have deprecated this, thus your 2.4GHz will provide a PHY rate of about 72Mbps tops. On the 5GHz channel you might get a 300Mbps PHY rate if all the stars align - good signal strength, 40MHz channel, two spatial streams.
But these are PHY rates, the raw communication speed between the wireless access devices. The actual throughput of data on these will be anywhere between about 50% to 70% of this. So this is likely to be about 45Mbps on 2.4GHz, or 180Mbps on 5GHz - this is about 60%, with everything working wonderfully on a gateway/wired AP.
Take these bandwidths, spread them across all the devices trying to connect in the RV park (since wireless is a shared medium) and allow for the 50% bandwidth loss as you traverse repeaters, and you soon get to small bandwidths.
So what could you look at doing? You could make sure you’re achieving the best link speeds between the access points. With the APs you have this could be through 5GHz sector antennas pointing back to the gateway (check the link speeds between the APs in the mesh) - although that would then mean you’re looking to push people onto the 2.4GHz spectrum for connectivity. You could look at replacing the APs with more modern devices which provider greater throughput (you can use the same licenses, these are the same across the entire MR range). But ultimately the biggest improvement will be if you can get more of the APs wired in rather than using mesh.
Hope this is of some help, and good luck.
Edit: on re-reading this I may have been a bit unfairly harsh on 2.4Ghz. Technically with two spatial streams you could achieve 144Mbps PHY on a 20MHz channel, so a throughput around 85Mbps. But that said, I doubt you’d get a good two spatial stream connection on 2x2 under 802.11n.
