Yep - low point drains. Assuming you are parked fairly level and assuming your unit was assembled with plumbing lines in a somewhat normal orientation, removing these caps (and opening faucets to release vacuum) will allow almost all of your hot (red cap) and cold (blue cap) water to drain out.
You probably have a third similar drain somewhere under there for the fresh water tank.
I was going to refer you to some good winterizing sites, but I see you are in FL so not sure what to recommend as I'm in the currently very cold Midwest.
The hot water heater drain that you mentioned is important too, but it is really only for the heater itself, not the rest of the supply and feed lines.
Brian, Stacy, and the girls (8 and 5)
2011 F-250 CCSB 6.7L PSD 4WD
2013 Solaire 269BHDSK