Blowing fuse

Dead short in the nichrome element comes to mind

Giving a hint of what year, brand/model of RV (and water heater) you've got would help immensely in trying to determine where your fault might be.

All I know from your post is that you replaced a "fuse". Because breakers have been used for so long on the 120v circuits, the fuse must be in a 12 volt circuit. That make sense because more than one appliance quit when it blew. The 12 volt is used for the control panels of the air conditioner and the water heater. The fact your problem occurs when you turn on your electric water heater element sounds like the separate 120v circuit to the water heater is somehow shorting out the 12v to the water heater. Open the water heater and make sure there's no sign of any wires touching each other. This is just a guess based on the information you've given so far.

I've edited the original post with the make / model of trailer and water heater.

JWP127 wrote:
I've edited the original post with the make / model of trailer and water heater.

Is the fuse you're replacing for the 12v side of things?

This page from your Suburban water heater's manual shows the 120v electric heater is completely separate from the control panel of the gas section. If turning it on blows a 12v fuse, there must be a cross connection somewhere.
https://www.manualslib.com/manual/601578/Suburban-Sw6d.html?page=5#manual See the lower left part of this page for the 120v electric element circuit wiring.

MEXICOWANDERER wrote:
Dead short in the nichrome element comes to mind

NO. not the problem that's a 120 volt device and trips a breaker

HOWEVER a dead short in the relay that turns it on and off......

but not the element.. the relay coil or the wires to the control board or relay.

2019 model. Still under warranty ?
Take it to the dealer.

Your problem is with the 12 volt control circuit in the Wh control board. Dead short. Furnace and Fridge my be effected as well.

The inside switch for turning the WH on electric is 12 vdc that operates a control relay to supply 120 vac to the WH circuitry and heating element. I believe that control relay is the only element common to both 12 vdc and 120 vac. Relay is typically mounted to the side or back of the water heater.

But the problem may very well be that the circuit downstream of the switch is shorting out. Possible the wire to the relay has been damaged and touching ground somewhere between the switch and the control relay. The A/C thermostat uses 12 vdc. And if the fuse that supplies power to the WH control switch also supplies power to the rear A/C thermostat that would explain what is happening when the fuse blows.

The upper diagram shows the 12 vdc to operate the relay. The lower diagram shows the relay supplying 120 vac to the small black switch on the water heater.

might be a dumb question since it was on day 3 but is the water heater full of water if not then it would be the element burned out and blowing the 12 volt control fuse at least that is what happened to mine several years ago when we were dewinterizing the trailer the electric water heater switch accidentlly got turned on and it only took a matter of a few seconds to take everything out.