There should be no problem mounting an RV power inlet on the underside of a horizontal surface whatsoever. The plug and connector are twist lock and on top of that, there is a locking collar that physically holds the connector on the end of the cord to the inlet. There are two types of collars - one that has a 1/4 twist turn and one that is threaded. I would go with the threaded type then it will be really nice and secure.
I think you'll find that a Marinco inlet is pretty good. We had a Conntek one on TT and it would not thread on at all but our Marinco cordset worked just fine on the Conntek inlet. Marinco cords are yellow but you could always buy just the connector and install it on a black cord.