Are you looking under the year and model of the E350? it may be 1-2 years older than the year of the motorhome. It will probably be on the driver's door post under the striker and will say "incomplete vehicle manufactured by Ford Motor Company".
If you go to one of the Ford dealers that sell parts on-line (my choice is Auto Nation White Bear Lake Ford) you may be able to get a Ford PN that you can search for a cross reference on.
Bill AKA "LOBO"
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"Getting old is inevitable, growing up is optional"
Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator
Wife's 2011 Flex Limited
Daily Driver 2009 Flex Limited with factory tow package
Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413