You're on the right track. Yes to sealing the floor. The thirsty plywood will suck out the more volatile binders in the adhesive before they can chemically react, and diminish the quality of the bond. You can seal it with anything sold as a “sealer”, or any mixture of left-over paints/polyurethane/mis-tints that you have lying around. The adhesive prefers a hard, cured, impermeable substrate. No need to sand too smooth, as a little “tooth” will help adhesion.
The coefficient of expansion of flooring material in a trailer is a weighty issue. Appropriate trailer flooring is a very arguable subject. You will get hugely conflicting advice... No expense spared to what's the cheapest and easiest. Unfortunately, “Easiest” seems to be a primary concern. Cheap vinyl may invite trouble, whereas Marmoleum appears better trusted. Children, pets, temperature variations, resale, longevity... many factors to consider. Research sheet-goods thoroughly. Some flooring will crack over plywood seams.
Like my house, my trailer is a “no shoes” area, so engineered maple, glued and nailed, was my choice for elegance and durability. I'm a big fan of the glue-down, don't like floating floors. Personally, I would spare no expense on the floor.
There are many links to flooring experiences... I responded to the following post with my successful experience.