Check your code to know at what points you need to anchor your cable. Within a box, there can be a clamp that is screwed in like here (it’s not yet secured properly). Plastic boxes usually have little plastic flaps that help to hold cable at the knock out hole. You will also need to secure the cable near where it enters the box and periodically along the run to the next box. Given the vibrations that tiny houses on wheels are subjected to, err on the side of plenty of anchoring so wires won’t pull or work loose. You can use staples, being careful to not damage the cable when you pound them in.