Hey John,
I don't have any water based sealer recommendation. However, seeing this went most of today day without a response I thought I would throw in my two cents.
I'm guessing you mean sealing the framework before you seal up the floor structure.
First a full disclosure, I do not have a water system in my trailer (potential for leaks). And I live and travel in a climate that is moderate. And I am not trying to discourage you from doing what you can to prevent water damage. If it feels right do it. BYOB!
After seeing your latest photos in your build journal it looks like you will have a 1X structure that will be wrapped in plywood of some sort. On my woodie I used 1/8" lauan on the underside and coated it with black jack. Yup, lauan. And I did not seal the wood framing before adding the bottom and top plywood. Irresponsible some would say. This spring (six years later) I went under and did an inspection while I was touching up some tar on my son's foamie build. There was no evidence of water intrusion (visual and wood moisture meter), very few areas where I added a smear of tar and only evidence of a little road dust.
I accept that some folks will poo poo the old fashioned idea of coating the bottom with tar. That's OK. I am only suggesting that the underside of your cabin will likely not suffer a great deal of water damage with the exception of the edges along the walls. Adding a drip edge to the underside edges will do wonders for moving water off the surfaces. Any product intended for sealing the under surface should be enough. Tar, The Mix, rubberized roof coating, etc. They have all shown to work well. Just pay attention to the bottom edges and try to prevent water from dripping off the walls and sitting on the area under the floor, especially near and behind the wheel wells/fenders. If you lived or extensively travel the bayou country of the gulf states or PNW I might suggest a different approach.
Look through some of the repair/overhaul builds here and you might notice that the rot is concentrated in other places. Undetected leaking plumbing, cracks in trim along exterior corners, wheel wells, windows, doors. And the real horror stories are on a lot of flat roofed, white box manufactured RVs owned by people who not only didn't build the RV but do very little to no preventative maintenance or leak detection until its done substantial damage.
I don't know you well John, but, you are building your baby. I suspect you will be taking care of her. You are not in that category of folks that just hook up and drive.
OK, I'm done ranting