The pair of levels are a great value.  I recently purchased these for a second time, as the originals were sold with my previous trailer and I wanted to make my new trailer easier to get on a level stance.There really isn't much to go wrong as far as product failure is concerned.  Accuracy is up to you with the initial installation.Recommended method of installation is to get your trailer level/RV, front to back, and side to side.  Once you have your trailer true, install the graduated levels in locations which will show you front to back level (placed on the side of the trailer) and side to side (place on the front or back of the trailer).  Make sure the bubble indicates "level" by being in the center point of the tube.Installation should be done once all your components are "stressed" as they will be for trailer/RV use (I.E. front tongue on a travel trailer, all the normal weight sitting on the winch foot, or in the case of 5th wheels/motorcoaches, landing gear down), try to place the levels on the RV in clear view of the winch/landing gear controls to enable leveling without having to relocate to a different location to view the levels.I accomplished this on a travel trailer by placing one level on the very front center, and the other on the side of the frame tongue close to the winch.  Though small, with sharp eye-sight, you can take advantage of the one on the front of the trailer as you position the trailer to see if you can establish a level side to side stance as you near your desired location.On my 5th wheel, my landing gear controls are on the front corner of the main body.  I placed one on the side of the rig for front to back level, and another right around the corner on the front side (for side to side level) still within sight of the controls, as well as from the truck cab for backing up.  Even though the level is off center, if it is installed properly with the trailer level at the time of installation, it will work fine.As far as the graduated scales are concerned, you will have to find your "scale" for adjustments, as it depends on where the levels are placed in reference to the center of the pivot points.The levels come with a self adhesive foam strip on the back for placement as well as two holes for screws if you wish to drill holes in the side of your rig.  I go with the adhesive strip route, as I don't like unnecessary holes.  The only negative with the adhesive strips is they will flex if stressed and it can put the levels off level, though this only happened to me with the first pair after the foam tape aged a few years.One last comment . . .the green bubble tube will fade from extended UV exposure.  It will not interfere with the accuracy, just makes it a little harder to read.If you are looking for larger levels which can be read from a distance, I know they are out there, but I have no experience with them to make a comparison.  I do not wish to clutter the appearance of the rig with large levels.