Depending on what kind of a joint you make, splicing can be perfectly fine. A properly made scarf joint, for instance, will be as strong as the base plywood. Making one would require the construction of a jig or two of some sort, or using one that someone else has made.
A scarf joint is basically just a very shallow angle wedge or miter sort of joint, typically something around 1:8 or 1:10 rise to run. For 1/4" plywood, it should be possible to cut the angles with a Skilsaw and a suitable jig; though I'm speaking not from experience but only from thinking it through as I'll also be replacing my motorhome's roof this year.