cliffn provided good links. They're what I followed back in 2016.
You'll need a Florida mail service. As linked, I like St. Brendan's Isle. They've been good to us.
We first filed an "Affidavit of Domicile" for Clay County, FL. This was registered with the clerk of courts. Additionally, we started switching all of our mailing addresses to the mail service (banks, insurances, cell phone, etc).
After that, you'll need to do the registration on your fifth wheel as it's your home. We didn't want to navigate the small DMV parking lot with our big rig, so we had a notary or sheriff (I honestly forget!) do a VIN Verification.
Once the fifth wheel registration is complete, you can then register your truck.
After both are done, we then got our driver's licenses. Our address is funky- it shows the license plate # from our fifth wheel (aka our home). You get weird looks from folks who need to see your license but no one has refused it.
Florida had a helpful website for determining what documents you needed to take:
What to Bring - Florida Highway Safety and Motor Vehicles
We went to the DMV office near St. Brendan's Isle as we assumed they would be used to dealing with fulltimers and specifically our mail service. The woman we dealt with couldn't have more kind and friendly. We literally laughed and joked the whole way through our time there. Took maybe 45 minutes from start to finish.
Easy peasy.