I don't believe there's any way that the heater could/would melt the water tank, those tanks are very heavy duty. If anything, you would blow a fuse or burnout the heating element. Personally, I wouldn't worry about it, just test it out to be sure it's still working the next chance you get.
Now, if it had been your hot water heater, be ready to replace the heating element!
Days camping (2015)----88 days
Days camping (2016) ---110 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi
30 years RV'ing
12 different RV's