No AC unless it's small. MW OK, 2 refers OK, furnace OK.
There is a device that will combine the two plugs in a standard household plug. This provides more contact area. No way to get 30A.
You'll also want to monitor the voltage with a DVM as you may have low voltage. You'll be limited running multiple items including electric HW.
Also is it a 15A plug as in one of several on a 20A CB? If so then you are even more limited. Hopefully it's a 20A plug on a dedicated 20A CB. Plus if it's GFCI then you need to check the rig and eliminate any ground faults.