I have tweaked the dosboxKQ6.conf file to match the original experience.

The game doest start in full screen (I find full screen really annoying)

If you would like to open it in full screen on start look for the below text

[sdl]
fullscreen=false

change false to true, save the file and it will open in full screen at start up.

rename the file dosboxKQ6.conf in your C:\Program Files (x86)\GOG.com\Kings Quest 4-5-6\Kings Quest 5
directory to dosboxKQ6_BAK.conf

copy the below text and paste into notepad.
save the file as dosboxKQ6.conf
copy the file to directory C:\Program Files (x86)\GOG.com\Kings Quest 4-5-6\Kings Quest 5
directory

and enjoy.

If it doesnt work for you rename dosboxKQ6_BAK.conf back to dosboxKQ6.conf and replace the moded file.

Copy from below the hash's
########

[sdl]
fullscreen=false
fulldouble=true
fullresolution=original
windowresolution=original
output=ddraw
autolock=true
sensitivity=100
waitonerror=true
usescancodes=true
priority=higher,normal
mapperfile=mapper.txt

[dosbox]
machine=svga_s3
captures=captures
memsize=4

[render]
frameskip=0
aspect=true
scaler=normal3x

[cpu]
core=normal
cputype=auto
cycles=6000
cycleup=5
cycledown=5

[mixer]
nosound=false
rate=44100
blocksize=2048
prebuffer=10

[midi]
mpu401=intelligent
mididevice=default
midiconfig=

[sblaster]
sbtype=sb16
sbbase=220
irq=7
dma=1
hdma=5
sbmixer=true
oplmode=auto
oplrate=44100
oplemu=default

[gus]
gus=true
gusrate=22050
gusbase=240
gusirq=5
gusdma=1
ultradir=C:\ULTRASND

[speaker]
pcspeaker=true
pcrate=44100
tandy=auto
tandyrate=44100
disney=true

[dos]
xms=true
ems=true
umb=true
keyboardlayout=US

[joystick]
joysticktype=none
timed=true
autofire=false
swap34=false
buttonwrap=false

[serial]
serial1=dummy
serial2=dummy
serial3=disabled
serial4=disabled

[autoexec]
# Lines in this section will be run at startup.
mount C .
c:
cls
sierra
exit

Your original experience was playing it in a window? ¬_¬