JohnnyB's photos are the way to go..
Ask for "LATH" strips at places like Home Depot or Lowes and even some lumber yards may have them..
So what is LATH (lathing) ?
it's the 2" x 1/4" boards (wood slats) that are nailed to wall studs (sometimes used with chicken wire) to hold the plaster mud in place when building a plaster wall instead of sheetrock or paneled wall.
Lath strips are thin enough to be flexible and will contour to the curve of a Van's side walls … and being "real" wood, it's thick enough and strong enough for attaching paneling securely so the paneling won't bulge out or pop away from the wall.
Use PAN-Head "Self-Starting" metal screws or pan-head bolts to attach the Lathing to each metal rib of the van.
You should use enough of the lath strips to place them about every 6" to 8" apart starting at the bottom and working up to the roof line.
notes:
* make sure to pre-drill the screw holes in the Lath strips to avoid splitting the wood.
* use a large (1" or 1 1/2") flat washer under the head of each screw or bolt so as to spread out the holding pressure and prevent cracking or splitting the wood.
* if using bolts be sure to use "STAR" locking washers under each nut so vibration won't loosen them.
*** regular "split" lock washers are great for non-moving parts, but vibration can cause them to loosen.
With this type of setup you can use screws and hooks instead of glue or tape to attach things like shelving, light fixtures, fans, etc.. or other things you may want to attach to the wall.
While there are plastic strips the same size as Lath strips, I don't think they would hold up in the hot summer weather when your van sits in the sun for several hours.. The plastic strips could (?) get soft enough to sag under the weight of the paneling or the screw heads may "pull-through" the heat softened plastic strip and you are back to square one having to do it all over again.... with twice the work involved.
As always folks, this is just the way I built my van 40+ years ago .. but it does not mean it's the only way.
Your way may be as good or even better than my way, so to each his own.
Best wishes on whatever you decide,
John