Have you checked to see if Dometic has a unit that is about the same size as your old one? It is ALWAYS best to cut the top. The bottom, the floor of the refer cabinet and such is supported and you will have to cut out the floor and drop it and then make and install supports for the floor and then seal it. Do you have roof top ventilation or sidewall ventilation? THAT will also be a factor in determining going hi or low. If you go HI and the top of the refer condenser coil is higher than the sidewall vent, the refer will not vent correctly. Doug