It's probably way more effort than it's worth, but a competent electrician could install a 30TT outlet in place of the welding outlet. They would also have to go into the house subpanel and replace the welding 240 volt double pole breaker with a 120 volt 30 amp single pole breaker, while also turning one of the hot legs into a neutral.
All the wiring is there for this conversion, but it would REQUIRE a competent electrician who could do this all in about 15 minutes.
As stated, it's probably not worth the time and expense for just a few days stay.....since it would all have to be changed back when you left in order to use the welder.
You probably can still use a 30-15 amp adapter and plug into a regular household outlet at the b-i-l's. You just are now limited to 15 amps total, and probably don't need to use the air-conditioner.
2011 Flagstaff 831 RLBSS
I've just ordered a chicken and an egg from Amazon.... I will keep you posted.