So what can you fit into a Class B+ RV?

RV Prep

Fuel Up/Check DEF (deisel)
Fill Propane tank
Windshield Wiper Fluid
Check the Wiper Blades
Fresh Water in Tank (washing)
Bottled Water (drinking)

Tool Kit (basics are fine, don't forget the Duct Tape!)
Leveling Blocks & Chocks
Collapsible Hose
Heavy Duty Extension Cord
Surge Protector (exterior)
30/50 Adapter
Latex Gloves for Service Bay (you will need them!)

Black tank deodorizer
Biodegradable toilet tissue
Hand Sanitizer

Clean Interior and Exterior
Check Tire Pressure (very important!!)
Maps, Reservations, Notebook
Pens, etc. (I bring an office kit)
Flashlight and Batteries
Walkie-Talkies (2)
Rain gear
Knee pads (see check tire pressure above)
 

Kitchen

Dishware
Flatware
Glasses, Mugs
Pots and Pans
Cooking Utensils
Can opener (manual)
Measuring cups, spoons
Wine opener
Chef's Knives
Dish Towels, Sponges, Scrubbers
Dish Soap
Pot Holders
Plastic Wrap, Foil, Baggies
Cutting Boards
Koozies
Mixing Bowls
Wine Glasses (yes, glass)
Travel Mugs
Insulated Tumblers
Cooler Bag for Beach
Napkins, Paper Towels
Induction Burner (fires are overrated for cooking)
Glass Counter-Top Oven (I never said we were roughing it!)
Coffee Maker
Groceries for the Trip (I suggest making a menu to get an idea of what to buy. Don't forget spices!)

Bath/bed

Bed Linens and Blankets
Pillows and Cases
Throw Blanket
Towels for Bath
Towels for Beach
Shower Tote (for using the campground bath house)
Shampoo
Body Wash
Bar Soap
Toiletries (his/hers)
Tissues
Laundry Bag
Laundry Soap
Clothes Pins and Line
Wash Cloths, Hand Towels
First Aid Kit

Bio-degradable Cleaner
Disinfectant Wipes
Glass Cleaner
Broom (there will be sand)
Dust Pan

fun stuff

Inflatable kayak, paddles
Life vests
Water shoes
Camp chairs
Beach chairs
Folding tables
Beach floats
Snorkel gear
Bikes and helmets
Cameras (video & still)
Music
Chargers for Electronics
Ukulele, Guitar
Bongos?
Bluetooth speaker
Real Books and/or Kindle
Laptop (handy for watching movies, researching area)
Cooler (good place for drinks to free up fridge space)
Binoculars
Hiking Gear
Golf Clubs
Fishing Gear
 

The list is not exhaustive, but will give you an idea of how to prepare your own get away list. Oh, I forgot to list clothes! Maybe a bathing suit, some flip flops, shorts and tee shirts will do!  Actually, we use a "capsule" concept for wardrobe keeping the number of pieces to a minimum and try to keep everything mix and match. Not always fashion forward but very practical.