This is it, why you have a motorhome, somewhere to stay. It might have to serve several other roles as well but without this one then it isn't a motorhome.
In the UK there are rules for the authorities accepting a vehicle as a motorhome or motor caravan and while they can be dismissed as just rules, they generally make sense as to what you will want.
Odd requirement to state perhaps but it means that there will be a door to go in and out by with the priority given over to the accommodation space. Using the driver or passenger door doesn't count, but you can use the accommodation door to enter the driver and passenger seats.
For convenience this wins. Not always pushing past seats, you can get in and out without asking people to move etc. There could be a safety issue, such as getting out in an emergency, but from a practical point of view, having a door dedicated to living in the van is a good option.
Campervans without beds are called day vans. Nothing wrong with that and if that works then great. If you want somewhere to sleep then perhaps you have an awning or a separate tent. If you want to just pull up and put your head down for a good nights rest then a purpose built bed is ideal.
The immediate option is to build a bed in the van. This can use a lot of space up, but when it's done its always there, ready to use. To save space, permanent beds are often raised up to the top third of the van height and the space underneath used for storage (sometimes called a garage) or perhaps cramped living space. These can often incorporate bunks etc. We did see a large van with sleeping for eight in four bunks either side of the van rear and carrying nine people.
The other common option is to convert existing furniture into a bed, nearly always the seating, where support is positioned and the cushions are rearranged to form a mattress.
In some instances the floor is used, and even kitchen work surfaces adapted.
A third option is the fold out or drop down bed, where the bed is stored ready made, either held against the side of the van to fold down or lowered from the ceiling on pulleys or rails.
Yet another option is the hammock style, where the bed is slung from hooks, either as a traditional hammock or as a ridged structure.
There are lots of options, but all a trade off between space and convenience.
The biggest thing to consider, is that you have to sleep in it so if it's not comfortable, you don't have space, it's to hot or cold, then its not going to be a good bed and you won't be getting the best rest you need.
Everyone needs water and accommodating it conveniently in a van is nice to have. Realistically this could just be a plastic or metal can with a pourer, but small water systems are nearly always created with either a permanent or removable tank, pipes and taps, and a waste system.
The basic is would be buying bottled water from the shops and using that. There are a surprising number of people who do just this.
Plan for somewhere to eat, flop, relax, chat, whatever, but unless you are going to sit on the ground in the rain or use somewhere else, then it needs thinking about. At one extreme, the van is just a big fixed bed with, say, storage underneath and an external awning. The other might be a fixed dining area, fixed lounge with TV, and even a hall area for those with large camper-flats with the pull out sections. Most will be in between with reusable areas, bed at night, becomes lounge seating during the day, with a removable table creating a dining area when needed. Somewhere with a table is invaluable with children on a very wet day!
If a van is going to be used then it needs places to put things when they aren't being used, when you're travelling to stop them bouncing around, when they're wet, when they're dirty, when you're going to trip over them and when you want to keep them hidden. The basics might be just hooks and straps, which many vans are equipped with from the manufacturer, and are always useful. Cupboards, drawers, boots, trunks, garages all come in to play but need thinking about early on as they take up space which if you aren't using is just wasted space. Things to think about are the largest objects, the everyday use objects, food, water, transient objects (like clothes, footwear) and also wet objects, which will come into your van.
This varies tremendously as to where you are using your van, in the mountains in winter will probably always be indoors, summer in the desert, always outdoors, so what the kitchen will look like will reflect this. Indoor kitchens are usually made up of a hob, food preparation area, a sink/bowl for water and more frequently these days, a refrigerator/freezer and microwave oven, pretty much what you get in a domestic kitchen. Grills and ovens are also installed but not as often. Cooking with gas is very common, though all electric installation systems are used, especially where EHU is available. Fixed installations are the norm in temperate and cold locations, but where outdoor living is normal, pull out kitchens are built, allowing the kitchen to be used outdoors, and then stowed inside the van after use, and some of these allow use both in and outside the van.
The stealth van is an aim of many, having accommodation which doesn't look like it is. But for many, such accommodation is claustrophobic and unpleasant, needing a window to let in daylight and look outside. The other extreme of this are convertors who use buses as starting vehicles where most of the vehicle is window and some have to be blocked up for practical reasons. It's a personal choice, but one which can be difficult to remedy one a build is underway, though where a base van with a sliding door is used, the sliding door can usually accommodate a window without difficulty later.