Shouldnt need more than a 2kw even in a landy (we run a 2kw eberspacher in the transit and ill be fitting the same in the motorhome ...
folks often get problems with fitting too large a heater ...larger than needed meants they often dont get up to full working temps and suffer from sooting up .
well worth running a gallon of kerosene/paraffin through them once a year or so just as a service (kero/paraffin burns slightly cleaner/hotter than diesel and thus helps to burn any carbon build up on the glow pin/burner mesh .
inlet AND exhaust silencers are well worth fitting as they reduce combustion noise by @50% imho ....setting to a sensible temp also minimises noise AND current draw as it reduces the amoint of times the heater goes through it's initialisation run (the noisiest bit) .
I truly wouldnt be without one in the camper or motorhome ....
2kw no good in ours due to large hole on roof with tent stuck on it