You don't mention what you are going to mount, but, there is a better (to me anyway) way to attach things to the walls that don't require drilling the wall.
Command Picture Hanging Strips
I use them to hang pictures, clocks, temperature gauges, towel holders, toilet paper holders, etc. and have never had one fail. They aren't your usual velcro strips; they are little plastic "mushrooms" on both pieces that interlock and just won't let go. They have the usual Command adhesive that sticks to almost everything forever, but, are easily removed if you need to. I also use them to hold my table lamp onto the end table, the alarm clock on the dresser and a fan on the dresser at the end of the bed for when I don't want to use the 2nd A/C unit (it is mounted in the vent that would normally be over the bed...). If the item is really heavy, just add more strips to the edges. I've never had anything fall off the wall using them. They keep the item secure, don't allow the item to swing (marking the wall) and yet, you can pull the item down to put new batteries (clock, temperature gauge) in when needed and if you don't like where the item is hung, you just release the Command adhesive strip and move it.
Just another alternative to drilling holes...
As for the ladder... I put a post somewhere on here about trying to put a ladder on a trailer that doesn't come with one. Has to do with the wood blocking that needs to be in place for the ladder anchor stanchions to go into. The external filon is not structurally sound enough to take vertical weight without eventually causing the holes to go "egg shaped" and that allows moisture to enter the filon causing it to bubble and disintegrate. Do a search on this forum for my full post and the cautions involved. Basic rule of thumb; if the unit didn't come with a ladder or at least have the option to add one (even then, your wall might not have the supports added unless you ordered the ladder option), then your best bet is to use one of the compact extendable ladders and not opt for an "aftermarket" wall mounted ladder.