OP - Is your galley on the curb/passenger side? And the sink immediately to the rear from the entry door? If so,
This Illustration has the GFI right at the entry door. Doesn't make sense for it to be IN the sink base cabinet, but the drawing makes it seem like it could be.
It's of course possible somebody had trouble with the GFI and changed it out using a non-protected outlet. My belief remains that there IS one and you haven't found it.
If I recall, WinTasca had at least three "31-foot" models in/around your year. Time to get a flashlight and go hands/knees. Inside/outside/under/above/behind...
Heaters are ROUGH on RV's!!! Nearly all of us use them but their usage isn't the same as at home. First off, every one is running on an extension cord. What? Yep, look on the ground between your coach and the campground pedestal. With that usually comes low voltage. Your heater's rated in WATTS. Less Volts, those Watts want more Amps. More Amps is more HEAT. And I'll venture nearly every RV is wired cheaply compared to nearly every house.
Rock, only outlets I've had to replace that were scorched like that were GFI's. I wonder if their internals are such that they can't handle their rated load on the downstream terminals... Then again those two might have come from a big box tool store with initials "H-F".
For others watching, bear in mind, there are 15-amp and 20-amp GFCI Receptacles.
Here's a 20. Notice the sideways extension in the longer plug slots
This one's from an Amazon seller called Topele. Made in China like everybody else but lists a few upgraded features for weather resistance.
Note to Self - Replace GFCI's used in High Demand circuits with Name Brand 20A. 20A won't upgrade a 15A circuit but just might last longer.