As others have noted just use the basic dog bone adapter (30 amp plug on one end 50 amp outlet on the other.. Or there is another one that is 1' long has a 30 amp RV plug (TT-30) and a 50 amp marinco outlet to plug into the RV's outlet.
50 amp service is 120/240 volt (2 legs 120 volt each basically a 240 volt transformer output center tapped).
However less you have an all electric rig, and possibly even if you do, NOTHING in the rig needs 240 volt with the possible exception of an energy management system, and it only needs it to detect when you are on 50 amps so it knows to "Stand down" and let everything work at once.
The 30 amp service is just 1/2 of the 240 volt (one leg) service so it is 120 volt. What the dog bone does is feed the same 120 volts at 30 amps to BOTH legs on the 50 amp outlet.. This is safe, and it works well, and in fact is how I am connected as I type this.
There is no need for a transformer.
There is, however a very strong reason to NOT USE any kind oftransformer.
Effiency
Use the dog bone and you get 30 amps peak, about 27 amps continuous.
Use a transformer and you get 27 amps peak, about 24 continuous due to losses (10%) in the transformer.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times