Yes, know what you mean.Think about what you need first when arriving on site eg hook up etc. We keep all these related items in a plastic bin with lid, found in lots of shops.Easy to pull out of back and wet things stowed away on departure.Back cupboard has towels/spare T's and pullies/undies.(Rolling things up takes less space).Overhead back cupboard has small TV (in padded bag)with remote etc, and other small useful spares eg sewing kit/medical kit/travel hairdrier etc.Wardrobe for hanging items (depending on where you're going and what you plan to do. Nylon drawstring laundry bag sits in bottom.
Under hob/sink cupboard (left) houses crockery/glasses/electric kettle/toaster/containers with tea/coffe/sugar/other tiny containers with spices/herbs/small salt & pepper mills/measuring jug/timer etc. Cupboard (right) cutlery & knives etc in tray.Placemats/napkins/foil/clingfilm/paper towel/tea towels/kitchen hand towels etc.Wok/pans X2/small kettle/wash up liquid. Room for Remoska when required on trip and food.
Underseat draw--warning triangle and kit that came with Cali/shoes/flipflops etc in bags.Spare loo rolls.Also food items--tins etc.
Have recently purchased a pouffe storage box from Aldi (black or brown £14.99) which houses the small Qube 335 loo.Am modifying it to open at one side so loo can slide in/out.(if anyone is interested get in touch and I'll write a detailed account)The box will act as a seat/footrest and fits into the gap left when the downstairs bed is pulled out-- for nighttime use or on route.Nearly forgot--bedding ( we take spare single quilts and pillows from home) fold up on back shelf along with fleece under blankets and sheets which I have made to fit top and bottom beds. We also travel with a parrot (surprise, surprise) in a travel cage which sits on the bench seat and is securely attached to the seat with bungees. The seat gets covered with an old cotton bed cover and towel, which acts as night time cover. Parrot's food/toys in padded bag to contain rattles.
Also have a fold up ladder which is housed in the back during transit and I keep the insect screens down the side of the back shelf but under the top mattresss, as some suggest, is a good place too. External thermal screen cover lives in bag under back parcel shelf
I've probably forgotten something but hope this gives you an idea of what you can fit in---and there's still plenty of storage room left. Our motto is--if it's not needed don't take it AND find a place for everything and put it back when you've finished using it.
Best of luck.