mjhjmtu

Here's a MWE:

documentclassstandalone

usepackagetikz
usetikzlibrarydecorations.pathmorphing

begindocument
begintikzpicture
 draw[decorate, decoration=random steps] (0,0) -- (10,0);
 node[decorate, decoration=random steps, draw, fill=yellow, inner sep=0.5cm] at (5,-2) ;
endtikzpicture
enddocument

For a more complex picture, I wanted to apply the same decoration to all lines and nodes, and so I used "every path":

documentclassstandalone

usepackagetikz
usetikzlibrarydecorations.pathmorphing

begindocument
begintikzpicture[every path/.style=decorate, decoration=random steps]
 draw (0,0) -- (10,0);
 node[draw, fill=yellow, inner sep=0.5cm] at (5,-2) ;
endtikzpicture
enddocument

However, this results in an error: "I cannot decorate an empty path". I think I understand what the problem is: node ... is replaced with path node ... and we thus have an empty path.

But how can I achieve what I want, namely applying the same decoration to all lines AND nodes (let's assume there a lots of them) without having to specify it explicitly for each draw and node command?

This problem has been solved some time ago by Jake, I think, in this stellar answer. It is sort of the opposite of what you suggest: if you use path (<coordinates>) node..., it works.

documentclassstandalone
usepackagetikz
usetikzlibrarydecorations.pathmorphing
begindocument
begintikzpicture[every path/.style=decorate,decoration=random steps]
 draw (0,0) -- (10,0);
 path (5,-2) node[decorate,draw, fill=yellow, inner sep=0.5cm] ;
endtikzpicture
enddocument

EDIT: The picture looks a bit as if the left edge of the node was straight. However, that's just an accident, as can be seen from the following animation in which the random seed varies.

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarydecorations.pathmorphing
begindocument
foreach X in 1,...,50
begintikzpicture[every path/.style=decorate,decoration=random steps]
 pgfmathsetseedX 
 draw (0,0) -- (10,0);
 path (5,-2) node[decorate,draw, fill=yellow, inner sep=0.5cm] ;
endtikzpicture
enddocument

EDIT: And if you don't want to write draw,decorate over and over, you could just append this to the node styles.

documentclassstandalone
usepackagetikz
usetikzlibrarydecorations.pathmorphing
begindocument
begintikzpicture[every path/.style=decorate,decoration=random steps,
every node/.append style=draw,decorate]
 draw (0,0) -- (10,0);
 path (5,-2) node[fill=yellow, inner sep=0.5cm] ;
endtikzpicture
enddocument

apparently every path doesn't work as you expected. you still need to explicit say, which lines, shapes has decorate path:

documentclassstandalone

usepackagetikz
usetikzlibrarydecorations.pathmorphing

begindocument
begintikzpicture[every path/.style=decoration=random steps]
 draw[decorate] (0,0) -- (10,0);
 node[decorate, draw, fill=yellow, inner sep=0.5cm] at (5,-2) ;
endtikzpicture
enddocument

so, it maybe more handy (for shorter writing) to define path style for example as

DP/.style=%decorated path
 decorate, decoration=random steps

and than use as follows:

documentclass[tikz, margin=3mm]standalone
usetikzlibrarydecorations.pathmorphing

begindocument
begintikzpicture[DP/.style=%decorated path
 decorate, decoration=random steps]
 draw[DP] (0,0) -- (10,0);
 node[DP, draw, fill=yellow, inner sep=0.5cm] at (5,-2) ;
endtikzpicture
enddocument