There are 2 systems in use in motorhomes - the better built units use the over-the-range system and these are 30" wide - depth varies and you normally have sufficient room to go larger if you need to. They have the extractor fan and can be made to vent to the rear or upwards using a supplied filler panel.
As has been said, remove the false floor of the cabinet above and you will find bolts of long 6" screws holding it in place. When removed, the front will tip down and it will hang from a rear mounting plate - lift off to remove. They are HEAVY to protect the cooker with a cushion or two.
On older units, you may just have a loose unit in a cabinet - normally they are less than 30" wide and you will have to check the venting arrangements - some just recirculate through a filter, some exhaust outside.
