This probably won't help you, but I'll mention it anyway: One of the best ways to get rid of clutter is to walk away from it.
I was very lucky in that I had 5 months, while still parked in my driveway and living in my RV (impending separation from my DH) in which to pick and choose what would actually fit in my motorhome; to bring things in and take them out again, pack and repack, etc. In the end, I left some things behind in our S&B that I planned to come back to get, but then our house was sold and I never went back. It helped a lot to not be looking at all the things I might be leaving behind.
I also started digitizing everything way before I went FT. I have not one CD or DVD in here, no photograph albums, virtually no books, no more paper records of anything, and no physical magazine subscriptions.
All my bills were being paid online before I left.
If your packrat DW is unable to part with mostly things of sentimental value, you might as well resign yourself now to paying for a storage space somewhere in the country. There is absolutely no room for those inside an RV (unless they're tiny, like jewelry, or fit on a wall).
When it comes to hobby items, I decided what was important to me, and have both of the under-seat compartments of my dinette allocated to needlepoint and craft project supplies, and musical instruments. But I left behind a motherlode of luscious yarns and beads and other supplies, and those are gone now.
If any of the crap is kitchen crap (like a bunch of specialized appliances, or drawers-full of kitchen tools), you learn how to make a few items do the work of many. You get creative.
If her thing is clothing (or, god forbid, shoes), there will be very little room for that, either, especially with 2 people in one unit. Maybe a lot of that can go in storage, too?
The idea is that if she cannot see it, and learns to happily live without it because it isn't actually gone-gone, but just in storage somewhere, that may be the way to part with whatever won't fit in your future RV. Then, if you're both happy with the rig and the FT life you've chosen, after a year of paying money to store a lot of stuff you never use or need, she may be willing to just stop paying for the unit and let it all get auctioned off, or pay someone else to sort through everything and sell it on eBay.