Wow - how far off do you think you'll be on a manufactured home?
Are you also saying you cannot access the lot information to get size, 'or' the deed? That makes no sense - but some places don't have GIS I guess or online tax information. Even the bordering properties will have length measurements in our GIS when zoomed in

I have found many 'Mobile' Homes with no tax data - not required if they are 'Mobile', not attached to a permanent foundation. I have never been afraid of missing the numbers - just place data for subject has been estimated.
Pull all comps up and look at them to get a better idea - then just do it.
Are you doing a BPO? Broker Price 'Opinion'?
Easy for me say - I have no qualms about estimating if that is what I am asked to do or if it is required.