The instructions for Victron do allow horizontal mounting though vertical is preferred. We have ours mounted horizontally in the Roadtrek and it supports high usage scenarios like running our air conditioner for over two hours or doing a complete charge on our lithium battery without coming close to the derating spec or overheating.
In our Newmar we found it was important to ventilate the compartment. The size of the two compartments (totaling about 48" full width pass through over the chassis rails) was just too small for the inverter to support high usage (like the above mentioned scenarios) without overheating.
This was with a baffle to prevent the inverter from recirculating warm air. We added a couple of strategically placed dryer vents and a thermostatically controlled 12V fan to insure the inverter stayed at a reasonable temperature.
In both coaches the wireless temperature sensors I had installed in the inverter compartments were invaluable, I did not have to depend on the inverter complaining about overtemp and could keep the temps lower than the derating temp specs on the inverters.
Stewart, Brenda and kids
2008 Newmar Canyon Star 3410, now at a new home
2006 Roadtrek Versatile 210