Makes a mess but easy to vacuum or sweep. The trick is to get all the old finish off and sand lightly. Any white streaks will show thru, so be sure the wood looks even. Amber shellac will cover a multitude of sins but your trailer is a lot lighter as my Deluxe was. Clear is closer to the original finish.
Hopefully others will chime in on the varnish issue. I thought mine was shellac because it scraped so easily.
I would guess clear shellac. I sanded all of mine off, it is an terrible job and took me days and then I re-coated with an amber - clear shellac combo and I still got some streaks in it from the old shellac but I think I use a gallon of clear and a quart of amber and that was a nice compromise. My problem was the old shellac yellows and you can never seem to find a match.