Our old Salem Lite had a small sink and cabinet right up within 1/2" from the tub it was maybe 14" wide. I'll have to look and see if I have any pictures of the old rig. It was tight, but very usable. Problem is, you really need about 16"-18" of leg room between the sink and commode. I would think there must be something out there small enough. Maybe look at the sinks from van conversions? They tend to be really small. Hopefully you've already figured it out. If not, hope this helps.
2001 Ford F-350 7.3 dually
2000 Salem T23 (sold)
2015 Vengeance Touring 39R12 (We love it!)