This really depends on the RVer, your current boondocking setup, and your needs. We choose to upgrade to Battle Born batteries first in order to use all of our appliances (coffee pot, instant pot, etc.) without turning on the generator. We also have a great onboard generator so we have a way to charge our batteries.
If your batteries are working well and you use low energy appliances you may choose solar over lithium. We recommend everyone assess their needs first and then choose what will improve your boondocking the most.
It is not required to upgrade both to lithium and solar. You can upgrade to lithium batteries only or keep your current batteries and only upgrade solar. Many people never upgrade to lithium but fill their roof with solar.