In what way do you mean "freshen up"? New paint? Cleaning it? Updating it? Please give more information so we can assist you better.
J, Start with washing any rugs or curtains. Scrub all counter space,and clean the screens and windows. Tidy up inside, and remove any clutter. This project is harder than a house. You have limited space. Try something on the small side for decor. Fill a vase with flowers, or hang a small picture, maybe something you use your van for. Fishing, camping, traveling, whatever you like, and if you can open the windows, refresh the air. Aloha!
What I've done is put an ozone machine in it, close it up tight and set it for several hours. Let sit closed for several more hours then air out thoroughly..
Clean it first, then if you want you could paint everything white to brighten it up
If you mean clean it up after sitting a while, start with opening all the doors and windows. We have a 43 foot 5th wheel. We put bounce dryer sheets under the pullouts when it is sitting so that the air doesn't seem to get so stale. If you have a fridge/freezer, open it and clean it with baking soda and water to wash, if it still has that closed up odor, wet a rag with vinegar and wipe the whole thing out and leave it open to dry. We always leave the fridge and freezer open after we have used it so that it doesn't sweat once it is shut off and it doesn't get moldy that way either. If the carpet is smelly, sprinkle baking soda on it and let it sit for a while before vacuuming, you can do the same with upholstery. :ight a couple of candles in a scent you like, that will help. Your camper van may not be very big, but it still is work to freshen it up. While you are at it, you could put new area rugs down and perhaps make some new curtains out of attractive sheets to add in some new color scheme to change things up. Get new placemats for the table going with the same color scheme and it will help it look like it is totally different for the seasons that you use it.