The E-series upfitter's package for motorhomes includes neither spare wheel and tire, nor a place to mount or store one. The 55 gallon gas tank on the E-450 uses up most of the space behind the axle, where one would expect to carry a spare.
A RV manufacturer supplying a spare for the E-450 C, standard or optional, needs to source wheel and tire similar to OEM, and create a place for storage. Winnebago built a swing-down rack between the frame rails of the rear extension. Tire is held against the floor of the basement storage space in the model I have.
A similar space should be available on your Jayco, unless used for fresh water or wast water tanks. A local welding shop should be able to fabricate a suitable tire mount. I'm not sure you'll find the right size space anywhere in basement storage, unless you have a "garage" space.
