You may have trouble finding the thin luan that most manufacturers use in their trailers... I may be wrong, but, I believe it is ~3/32". Most big box lumber yards may only carry 1/8" as their thinnest, maybe even 5/32", in the A/B or A/C luan (A being the smoothest, B,C & D being rougher) In 4x8 foot panels you may only find 3/16" as their thinnest. If you have a custom cabinet shop in your area or a custom kitchen shop even, you may have better luck at a place like that. They usually carry 3/32" and 5/64" luan for cabinet work. In a pinch, you can probably make 1/8" work for you, but, it is going to be stiffer and you might need to dampen (not soak) one side slightly to aid in the flexibility for following the curve of the ceiling (if any; some ceilings are flat - would be a blessing for you
If you have an RV repair shop in the area, they can certainly order the luan for you...they could even order it already pre-covered with the same contact surface covering use in trailers...
Unfortunately, we live in the boonies...no stores of any sort (17 miles one-way to get a dozen eggs
), so, I'm no longer real familiar with what the big box stores handle, hence my "suppositions". Fortunately for me, we live in a big forest area (my home is in the middle of a state forest) and local lumber mills are readily accessable.