I suspect you will find that the front seats are not standard Ford van eats. In all probability they were added by Winnebago as part of the RV furnishings. perhaps Winnebago Customer Service could identify the seat brand so that you would know what to ask for. Generally, though, stock seat covers for them will not be available, so your choice is either a sloppy fitting generic cover or have an upholstery shop make a custom set for you.
Something like this might work:
https://www.covercraft.com/us/en/product/rv-seatglove.SVR