Neighbour across the street multiple decades ago had a bus for RV purposes that he and his son converted to propane. Seem to recall that Impco may have been the carb that they installed. The whole project was a home brew because back then propane was considered "exotice fuel and didn't have much by way of support beyond forklifts. Both have passed on so no way to know; haven't seen the other son in????If you want to save some junkyard searching, just search eBay for "Impco Mixer" or "Impco carburetor". It shouldn't be too hard to get a propane mixer for most engine sizes, but a decent sized storage tank can be a little pricey.
Nick