125' Run 30 Amp Service Camper
URD cable is a hot mess
URD cable is made with the bad AA-1350 alloy wire, infamous for the aluminum-wiring scandal of the 80s and 90s. (this alloy is banned anywhere else, see NEC 310.106(B)).
That's because URD cable is made to be on the power company's side of the electric meter - and power companies play by different rules (because their wires are all outdoors and not buried in walls of buildings). Note that you're not running a service (pole to meter) ... you're running a branch circuit (panel to socket).
That dangerous AA-1350 metal will be a nightmare to get the terminations correct so you don't get a repeat of the 80s house fires.
Cheap doesn't mean stupid (much the opposite)
Cheapness is a grand thing.  But it does not mean we are morons.  We still think ahead and are smart about being cheap now to save money later.
I'm thinking hard on two scenarios: #1 wanting to run other loads besides the camper, out there today. Using power tools to work on stuff. Having a picnic area with power for grill etc.  #2 a future RV that will take a 50A/240V receptacle.  #3 fitting a subpanel out here if needs increase.
So I recommend you throw one more #6 Al wire into the trench.
For now, you share the neutral with the other circuit, and bring it out to a common 120V GFCI outlet right next to the TT30 trailer circuit.  Now you can just plug in anything you want and you don't need the trailer there as a 3000 pound &quot;adapter&quot;.  Back at the panel you use a 20A breaker for that circuit and a 30A breaker for the TT30 trailer, and handle-tie the two breakers.
In the happy future you get a big RV, you just change the recep to a NEMA 14-50.  Just like that and you're done.  Well change the breaker also, but that's $10.  Or you could install a trailer power stand which provides all of them: 14-50, TT30 and 120V GFCI at the same time.
All you need to do to enable that is carry a 4th wire... so 6-6-6-6 for instance.   33% more for the wire but that's a small part of total project cost.
Conductor size is just fine though
Generally I don't even bother checking the voltage drop calculators unless a run is over 160' (or 80' if it's 120V).
Normally you use #10 copper (or #8 aluminum) for a 30A circuit.  Given the distances we need 1 wire size bump, and that puts us at #6 aluminum, so that's just right.
Thinking about that larger future RV at 50 amps... #8 copper or #6 aluminum is the minimum size allowed for 50A if the wire is allowed to run a 75C (so not UF-B or NM-B).  Since it's 240V we're not far enough to need a wire size bump, so we're all set there too.  See how easy that was?
Cable/Wire Type
I see where people might recommend 2-2-4-6 since URD is such a hot mess. That's because when you get away from URD cable, #6 cables get hard to find in an all-in-one cable -- for instance, in MH feeder, the smallest made is 2-2-4-6 which is overkill and won't be fun to attach to a 30A breaker.  You'd need some fairly hefty Polaris connectors on both ends to adapt it down to some #10 copper for that final connection.
As it happens, USE-2/RHH/RHW-2 is available in individual #6 wires, quite cheaply. (it's more or less MH feeder, but separated). These individual wires can be directly buried. But they can't be directly run inside the house on rafters and in walls - inside the house they need to be in conduit. Or you could stop at a junction box and transition to a different cable type for inside the house.
The only snag I see with USE-2 individual wires is officially, the neutral is supposed to be white and the ground is supposed to be green since these are #6 or below. However if that size of wire simply isn't sold in colors, the inspector is sure to let you off the hook.
Buy your wires locally at an electrical supply house.  Big-box stores overcharge you, and Internet sales companies build a lot of shipping/handling cost into their prices. Electrical gear is too low-value and heavy for mail-order to work all that well.