My suggestion would be to:
-Continue to bristle brush the remainder of the painted surface,
-Get a good exhaust fan to suck the air out of the trailer and a good face mask
-Use cleaner vinegar to spray the interior in patches, allowing it to sit a few minutes and brush again (This is the best non-corrosive solvent for latex paint
which I think is the problem you are dealing with)
Once the whole surface has been cleaned and hopefully stripped of any remaining loose paint, I would check for are any raised edges and lightly sand the edges. But if what is left is fairly flat, the next step is to apply a good all-surface adhesive primer. This will ensure that the primer holds everything left behind together, will bond to the newly cleaned surface and if judicially applied level out the patch edges. After that, choose your new wall colour making sure that it is compatible with the primer.