What is the bottleneck for RV upgrades?

Wood Blocks run out before anything else on almost every RV level, so plan your wood before the upgrade. The Homeland base grows out of your RV as part of the RV park, which is why the same shortage also slows Homeland level-ups. There the wood appears as wooden planks, and it is still the material that holds you back.

The quickest single source is a new Homeland plot. Unlocking one instantly gives you a large amount of Wood Blocks along with Mineral Sand. If an RV upgrade is stuck on wood, check whether you have a plot ready to open before you start grinding.

The slower, steadier source is Homeland orders. Production and delivery orders pay out progression resources as well as Home Coins and stamps. If the Order Robot is only offering orders you don't want, you can refresh them. Orders and crops run on real-world timers, so check Home Orders when you start a session. That way you plant the right crops early and finish the orders before you log off.

What the wood is buying

The RV levels unlock things you will want. The Harvest Moon Festival opens at RV level 10. On the Homeland side, reaching Homeland level 15 rewards a mysterious Prismana egg. Both milestones sit behind the same wood shortage, so time spent stockpiling Wood Blocks moves you toward both.

Not settled: The facts only say wood is the bottleneck at most levels. Which RV levels need something else more is not settled.