Passenger and side airbags on vans are very often options.
Possibly likely to be found on factory motorhomes as buyers would expect that, but on commercials used for self-conversions I would be surprised if they are seen that often at all. Sometimes the airbag 'space' will be used as an extra glovebox, but sometimes dash is unchanged and it is an empty space behind the markings.
Wiring could be there under the seat and unused as same loom fitted and used/unused depending on options. I suppose it varies between vans and manufacturers, but I think airbag activation is based more on seat pressure pads rather than seatbelt being fastened (as you may not have a seatbelt on when a passenger perfectly legally if you have an exemption and the maker could not have the airbag disabled in that situation as could get sued!). I think it could also be linked to drivers bag as well - thats goes off, the passenger one goes off.
I suppose the "Seatbag ECU" is also the pre-tensioner ECU and the SRS light is also relevant to the pre-tensioner connection maybe?