If used on a single tire position (IE: Front) handling may be affected I would get the proper tire put on ASAP but safe to drive
If used on a dual tire position (IE REAR) might as well not bother to bolt it on as it won't be sharing the road.
Though you can play a 3 tire game (put the spare on the front move the front to the rear)
IF you have to use a different size spare remember ASAP also stands for Always Say A Prayer. (That is the level of I do not recommend this on a motor home.. I'd rather call the tireman and have him bus me out a new shoe.