How am I supposed to hook power to this food truck with a female receptacle?

Your gut is completely right: that setup is backwards and whoever put it together was planning to run a suicide cord. You never feed utility or generator power into a female receptacle. A loose male plug with exposed live prongs is a massive shock and fire hazard, especally on a commercial rig where you're constantly working around metal counters, sinks, and a steel chassis.

You need to rip that receptacle out and swap it for a proper **50A male power inlet** (typically a CS6375 style or a NEMA SS2-50 twist-lock shore inlet). With an inlet, the exposed pins on the vehicle are completely dead until you plug in a cord. Your feed cable from the generator or shore pedestal will then have a female end plugging into the truck, keeping all the energized contacts safely shrouded.

While you're swapping the box, trace the conductors into the main breaker. Food truck builders cut corners on this constantly. Make sure the feed is actualy sized for 50 amps—you want a minimum of 6 AWG copper on all four conductors (two hots, neutral, and ground). If they used undersized wire, replace the whole run now before you're pulling heavy current on your equipment.

Also check how the neutral and ground are terminated in that panel. Since the truck is essentially a mobile subpanel, your neutral bar and ground bar must be kept isolated from each other. Bonding them inside the truck panel creates dangerous stray current on the chassis when connected to shore power. Fix the inlet first, check the bonding, and verify everythign with a meter before you try powering up any of your kitchen gear.

Man, seeing other people build out rigs gets me so hyped! I am brand new to electrical stuff and just started planning a shuttle bus conversion, so seeing what crazy things previous owners do is wild. Quick question though—if you're pulling a full 50 amps on a mobile kitchen, what kind of generator do you even run for that without deafening all your customers in line? Are people mounting giant dual-fuel inverter units on the tongue/bumper, or do you have to go with one of those commercial under-chassis diesel boxes?

Beyond replacing the inlet itself, make sure you verify what physical wire type was used behind that box, because countless DIY builders run standard solid-core NM-B Romex through vehicle walls, completley ignoring the fact that continuous road vibration causes solid copper to work-harden, fatigue, and eventually fracture right at the terminal lugs. If you find solid wire, pull it out and rerun stranded conductors through flexible conduit, and please use an actual torque screwdriver when securing the new inlet terminals, since under-torqued set screws vibrating down the highway are a classic recipe for high-resistance heating and melted plastic.

Quick update — glad I checked the wire like suggested. Pulled the box off today to prep for a male inlet, and sure enough, whoever wired this ran stiff solid-core Romex trough the wall. Since I have to rip that out, what flexible stranded wire setup shoud I run from the new 50A inlet to the breaker box on a vehicle like this? whould SOOW cable work, or THHN in liquid-tight conduit?