No adapter should ever be used to take 120 VAC from a 3 pole 240 VAC
receptical. 120 VAC recepticals have a hot leg, a common and a ground.
Most common 240 VAC recepticals have three poles, two hot legs and a
ground. You can get 120 VAC from ground to either hot leg, but it's not
safe. A four pole receptical has two hot legs, a ground and a neutral.
It can safe to use an adapter to get 120 VAC from this type of
receptical, if you know what is there. I should note that many states
have now banned the sale of adapters for this purpose. The most common
connection for 240 VAC is 120-0-120, all in a single phase relationship.
In some cases the 240 VAC will be supplied from a three phase system.
The result can be 240 VAC with 120-0-120, BUT 208 VAC to ground!
Gerald says "A simpler adapter would connect both sides of the 240 volt
plug to the hot side of the 30 amp receptacle." Connecting both sides of
the 240 VAC together will result in a short circuit. I'm sure this was
just a typo, but don't try it. The resulting arc and/or fire will not be
pleasant. Also, an auto transformer larger enough to supply a washer or
dryer would weigh so much that it would not be worth the weight to bring
along. Remember that by going from 120 VAC to 240 VAC. The current
doubles. That is, if you want 20 amps at 240 VAC, then you must supply
40 amps at 120 VAC to get it. Better to just use the washer and dryer at
the park or step up to a bigger gen set.
Dave Beard