A pretty easy access point to check the 2 phases of electrical feeds is at your inside circuit breaker panel, by removing the trim surround (4 screws?) . Then you have access to the 2 incoming phase feeds, which are the big wires going INTO the mains breakers. There should be 120 volts AC on each of these phase feeds.
You can also easily check for 120 volt AC power going OUT to the branch power circuits involved at the feed wires coming out of the branch breakers.
If all of these points show that you have power at the breaker panel, my best guess is that you have wiring problems. if the trailer has travelled recently first guess might be the interconnect boxes under the slideouts. If you want to do some checks in the warm inside, start opening up the outlets starting with those nearest to the breaker panel working your way away from the breaker panel until you detect a loss of power. DO THESE OUTLET REMOVALS WITH THE MASTER POWER BREAKERS OFF, ONLY TURNING ON THE POWER AT THE BREAKERS TO DO VOLTAGE TESTING. You might flex the Romex cables while testing and see if power comes back on. These outlet tests are to check for intermittent connections on the daisy-chained outlet Insulation Displacement Connectors (think Scotchlok, or search this term on the forum). These have been VERY FREQUENTLY reported as having problems on the forum over the years. Much different that the wire-clamp screw circuit branch connections at the branch breakers - You hardly never hear of problems with them, although it is good practice to tighten the connection screws once a year. You can't re-tighten an Insulation Displacement Connector connection, and the connection surface area you are relying on is about 1/5th the connection area of a wire-clamp screw connection.