Typically, most "Back up" light switches are on the transmission, if I recall. I could be way off here but, that's what I've found in the past, several times. I don't have your model coach so I can't be more sure of my suggestion. Sorry I can't be of more help.
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '08 GL 1800 Gold Wing
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Sophie character, (mini Schnauzer)
You should post this down in the Workhorse Custom Chassis Motorhome Forum - iRV2 Forums forum where they deal with your chassis. Someone there can hopefully provide better info. The Winnebago wiring diagrams for your rig (Table of Contents) do not show the backup light wiring. Based on the brochure for your rig, with a Workhorse chassis you have the 4L80E 4 speed transmission.