for many years we've used a 50-Amp RV Box Adapter :
we use this box (ours is pictured below) whenever possible at sites without 50 amp service .... it works best if there are 2 30 amp receptacles available but we seldom find that to be the case .... most of the time we use it with one wire plugged into a 30 amp receptacle and the other into a standard 110 .... we've learned a few things from using it: 1) 30 amp + 20 amp power provided by this 50-Amp RV Box Adapter does not provide the same 50 amp power you get from plugging into a 50 amp receptacle 2) the box will not work if the 110 receptacle is GFCI 3) you have to learn how much you can run on the 110 leg, for example, our camper appears to have the dryer and water heater wired to that leg and if the water heater comes on while the dryer is in operation it will trip the 20 amp breaker in the CG's box .... all in all, we have found it to be a very worthy adapter
Jim