FL domicile fulltimer here. I use SBI. Yes, we had issues with addresses for voting. County election office wouldn't accept the PMBs. SBI resolved the issue by having us use the physical address for a marina they're affiliated with. We only use that address for voting.
I haven't heard of any commercial mail forwarding services in south FL. Before you commit to them, find out the address they'll issue you and ask if they've had any issues with voter registration. Then check with the county elections office.
As far as your DL goes, FL DMV treats us as homeless. They're supposed to require information on your RV listed as address on your DL. Not all DMV offices are up to speed on this requirement.
