I didn't have any problems, but the previous owner had registered it as an RV with the state of Oregon a few years before so I didn't have to do that myself.
The tabs were going to expire at the end of the month, and he had told me I would have to take it through emissions in order to get new tabs. When I went to the DMV however, they found he had mistakenly registered it as a gasoline-powered vehicle. Diesel vehicles don't have to go through emissions, so I was able to transfer the title and registration and get tabs right there no problem.
Here's the relevant ODOT page on registering RVs: http://www.oregon.gov/ODOT/DMV/pages/ve ... e.aspx#def
If you're having issues getting it registered as an RV I could ask the previous owner how he did it.