I looked at some photos of a Roadtrek class B on Google search. That's a tough one you're asking here.
We once had a situation where we were going to park our tandem axle travel trailer on a site that was very uneven side-to-side. It was about a 12 inch slope (really). I was fortunate to check the spot out way ahead of time, as this was a state park and we were camped there, and this spot in question was reserved about a month out.
The only option I came up with was "lumber"... lots of lumber.
At the site (It wasn't occupied when I did this), I measured the slope of the spot and took all those figures home with me. Then proceeded to procure lumber. I started with an 8 foot 12 inch wide 1 inch thick board (from an old barn, and those were the true measurements), and beveled the end. I took another one and cut it a few inches shorter, and beveled the end. I continued doing this until I built it high enough the trailer would be level side-to-side.
Stacking the lumber means ensuring back end of the stack remained straight, while the front end formed a ramp. My biggest concern with the board on top of the previous one would slip out while backing the trailer on the ramp. Or, if I did make it all the way to the top, the back wheel of the trailer would over-shoot and fall off the ramp about 12 inches dangling in air!
I ended up drilling holes through the boards one level at a time and used dow rods to pin them together so the boards would not slip when the tire started roling up on them.
Yes, this was a lot of preliminary work I did, getting the lumber, designing the ramp with solid wood that came out of an an old barn, aged and seasoned over 100 years and hard as a rock. Triming, cusssing, and leaving my DNA (blood and skin) all over that lumber, I finally succeeded.
Transporting the lumber was not problem since I had an 8 foot bed pick up truck towing a travel trailer.
The great thing was, even though it took a lot of preliminary work and building, the effort was an absolute success. Trailer rolled right up the ramp (scared the bee-hee-bee-jee-bees right out of me), but it did work, and my wife had a good set of lungs when she yelled "STOP!" to keep me from rolling off the end.
We were at that spot for almost 2 week, so all the effort was well worth it. (This was a State Park in Western Central Indiana).
I say all this, for a final thought. Building such a ramp as this is probably about your only option. Take the time to do it, practice with it at your home driving up on it, and then take the entire stack of wood to your MIL and leave it there.
FYI, I still have all that lumber, but have never needed to use that much on one side of the camper again. I usually carry the 4 - 6 foot length boards with me when camping at a new campsite (just in case), and ended up using them on many, many occasions since they were originally designed. I keep them in my garage, and when getting ready to travel, make sure I have enough to raise the camper at least 6 inches. I also have a variety of smaller one (2 foot lengths) that fit under each tire separately, or if parked on a slope where I need front-to-back level and need to raise the tires to make this happen (35 feet long, doesn't take much of slope to bottom out the front or back of the camper you know).
My suggestion ... build a sturdy ramp and bring it with you.
