30 amp & 20 amp to make 50 amp

Just in case...why do you need 50 amps?

50A isn't 20+30, it's 50A X 2 for an actual 100A. (because it's 240V @ 50A)

You can make an adapter that uses them separately for each side of the 240V but if the 20 has a GFCI it will possibly trip.

Dedicated 50 amp 120 connections exist only in industrial applications. Twist-lock connections. Newer household stove receptacles is where you are MOST likely to find the CORRECT 4-pin receptacle and son-of-a-gun if it isn't a 50-amp outlet.

I have seen far too many factory 50-amp circuits that over-burden one leg leaving the second with a light load.

Simply put, unless an individual uses care and monitors a patch 20-amp feeder, it would not be difficult to overload the 20-amp shore breaker and have it trip.

The "cheater" cord won't work if there is a GFI on the shore power supply.

The two circuits must be on different phases of the electrical source.

Re-read your post and think i understand better. Your asking about one of those adapters that use the 20 and 30 amp receptacle to power a 50A rig.
My suggestion is to skip it. They only work where the 20A has no GFCI (a direct violation of code) so not easy to find.
Another idea, use the standard 30 to 50 dogbone to power your RV and create a way to plug your water heater directly into the 20A recept at the pedestal. I did this with our last TT and it was very helpful.

If the water heater’s draw is your concern, follow Scott’s advice. I’ve also done that with a space heater.

the OP did not mention WH

but when i spent (2) month s at my brothers on a 20amp circuit

i but the everything i could on propane

MrWizard wrote:
the OP did not mention WH

My point was to run some load in your rv off the 20A recpt. at the pedestal instead of running everything off the 30A.
Could be a Wh, space heater, toaster oven, Margarita blender, etc.

ScottG wrote:

MrWizard wrote:
the OP did not mention WH

My point was to run some load in your rv off the 20A recpt. at the pedestal instead of running everything off the 30A.
Could be a Wh, space heater, toaster oven, Margarita blender, etc.

I use a RV Safepower
transfer switch to supplement my power when I'm caught with my 50 amp RV on a 30 amp site. I can run my 2nd A/C or Washer/Dryer circuit via the RV safe power transfer switch. I typically plug my space heater into the W/D circuit. The switch works with GFCI outlets.
The RV Safepower is a game changer. 30 amp site are no longer a deal breaker for me. Bring on the state parks.