No the thermostat is not adjustable.
At first I immediately thought you have a winterize valve in the wrong
position or maybe only partially turned in the correct position.
That happens all the time.
Folks complain they have luke warm water and it turns out to be a miss-positioned
winterize valve handle.
But you say the gas heat hot water is hot??
If that is true it might be a bad thermostat, maybe.
I'm not totally convinced but maybe.
IF you start with a cold hot water tank and the electric heater is off,
and you turn the electric heater on, you should be able to hear the element
"sing" in the tank. It sounds like ....... whales talking to each other or
it sounds like a pan of water heating on the stove.
It's not real loud but if it's quiet around you, you should be able to hear it.
IF that happens and the singing stops carefully pull the pop off valve
handle briefly. That is found near the top of your water heater tank outside
in the burner area.
Be careful and don't burn yourself. IF you pull the pop off and get luke warm
water out then I'll agree your thermostat is bad.
Here is a photo of both the 12v and the 120v thermostats.
They are located under the rubber "reset" cover.
In this photo the one on the left is the 120v one.
You'll have to figure out which is the 120v in your heater.
You can replace it for about $20.
They can be got from most any RV parts desk or online at lots of places.
The come in 120 deg F and 140 deg F.
Get the 140 fer sure.
Dig out your water heater manual and there should be a parts listing
there with the thermostat part number.
Google that and you'll find lots of places to order it from with a range