My RV wiring.

[Jimbo13]

I want to put a on demand water heater in my RV, I have a 240v 30amp receptacle for the dryer, this is the same breaker the Water heater requires.

Will it function properly if I pigtail the water, I don't need both plugged in at the same time. I know you can't legally in a residential construction but are there any legitimate safety concerns?

[HayZee518]

only thing that in a manufactured home [rv] the pigtail needs to be a four wire pigtail. the matching receptacle needs four wires. the green or bare ground is isolated from the line neutral and is connected to a clean chassis ground on the heater.

[pushkins]

Yes there are real concerns (IMO) Everyone always says "I'll never have them running at the same time" but what if someone does ?, you sell the RV next owner doesn't know, a friend uses it ? etc...etc... On demand means the dryer might be on and someone decides to use the hot water somewhere on board, like the kitchen, bathroom, shower etc...
Why not run a new line with a new breaker ?

[Jimbo13]

How could I possibly run them at the same time if the purpose of this short cut is to share the receptacle to avoid the possibility that they would ever be even connected at the same time?

[HayZee518]

The return posts are not meant to do that. But it's human nature to get lazy. Eventually it's gonna get tiring unplugging and plugging in the pigtails. A dedicated line for each is what it is gonna come down to. Anyplace on an RV is easy getting to from underneath. I did that on my 12X70 trailer when I put in an over & under washer/dryer.

[pushkins]

How could I possibly run them at the same time if the purpose of this short cut is to share the receptacle to avoid the possibility that they would ever be even connected at the same time?

When you mention "pigtails' I am thinking your wanting to tap onto the existing outlet and make another one fed from the same supply.
I doubt doing it your way meets code (in fact I'd bet it doesn't) but at least if you could ONLY plug one or the other in any any one time you'd remain safe.

If it were me I'd still be adding a new circuit, it's really not all that hard especially on a RV.

[HayZee518]

Pushkins. a pigtail is a temporary connection from a load to a supply using a plug/receptacle combination. dryers have pigtails, ranges that are not hard wired have pigtails.

[pushkins]

A pig tail is also a small length of wire, wire nutted onto an existing circuit and fed into a new outlet, it's often done here (not be me or any of my sparkies) to double up outlets just like the OP. That's why i made the comment.
I have a photo somewhere here of that exact thing done do wire two separate ovens side by side., we did a whole whole renovation and found it when we gutted the kitchen.