Allow me to interject a bit here. You guys are going WAY over the top, but I never say no to making something totally bomb-proof. IN fact, I used polyurethane sealant (Sika-Flex 221 which comes in a nice aluminum grey!) to seal the outer mounting flange of my RV-500 when I installed it, but then the interior of this component is 100% accessible for servicing, so just the outer casing is now bonded to the trailer wall.
The 'industry standard' and the process I use in the field, is a product called 'butyl tape'. This is a putty-like substance formed into a tape and is found on a roll at just about any RV supply store. I use it on just about any thru-wall installation like furnaces, water heaters, roof vents, etc.
It makes a nice water-proof seal, is really easy to use and makes the appliance removeable in the event it must be taken out to be serviced. This is the great benefit over sealing it in with polyurethane. Just be sure that you get 'butyl' tape and not the cheaper 'putty tape' which will get hard with age.