Just to close the loop....
The MI title had "motorhome" on it, so the WA DMV (after a few calls from the office) just wanted a stat dec saying it was not a motorhome, and had no toilet or cooking facilities.
So it was titled as a truck, using a value code for an E350 cargo van (my VIN is a "partially complete van" which doesn't decode to anything at the DMV), BUT they charged me use tax (ie sales tax) based on the sale price..... which I guess is what it is ($4k...). If I had bought it in-state from a used car lot I would have paid sales tax.
So I thought I got away with cheaper registration, but was hit by the weight fee, $77 per year. So maybe $20/yr cheaper than RV registration.
Ironically there was a woman there trying to transfer title of a "camper van" (either a Westie or an SMB!). I never used those words because WA has a "camper registration" for one that fits on the back of a truck. They got the whole thing completed and handed her a trailer registration plate, and she said "no, a camper VAN". They were using a razor to peel decals off the plates when I left...... her kids were screaming....
Anyway, I have plates and a registration slip now.