lets do some math, o.k. lets not!
lets use the handy dandy wire size calculator instead...
now lets look at the varibles:
120 volts, copper wire, single phase, 30 amps 50 foot run (assuming 25 foot cord on the trailer and 25 extension cord)
lets see what we get...
thats right, 8 gauge!
of course that is calculated for 3% voltage drop, insuring no damage to your ac compressor.
now, without the extension you would be o.k., with it, are you going to overload? probably not.
as suggested elsewhere, actually checking the circuit with a clamp on ammeter would tell you for sure how much load is on that cable with everything running.
also it all depends on what your voltage at the outlet is to begin with, the lower you start with the less you will have in the end. the load in the trailer is essentially fixed, the lower voltage, the higher the current draw will be. ohms law!
if it was my setup i would try to find ways to avoid use of the extension if at all possible.
-but- if you find yourself with no other way to get power to the trailer i would not hesitate to use it sparingly.
you are right on the edge of capacity.
just keep that in mind!