Spend the money and get an all-in-one charger/inverter with a built in ATS (automatic transfer switch). It will replace the existing charging system, but that makes life easier in the end. The only difficulty doing that, though, is that it depends on where your batteries are located. If they are like min and are in the front of the coach, this is not the best solution (too far to go from the rear of coach to the front with 12V mega-cables that would be necessary for such a long run. That is why i left the existing charging system in place and put my inverter in the front electrical compartment. It is easier to run the live 110V wire back to the ATS under the bed than the other way around with the 12V cables. But, because my system already had an existing ATS to flip between shore and the genset, I had to cascade in a new ATS for the inverter.
On the topic of weather or not you need a true sine wave model:
My advice is get one. My first inverter was not and I had issues with one of my TVs (its flyback trans. would not charge properly) and then alos my laser printer and one of my computer power supplies did not like the mod sine wave AC. So, I switched it out and I never look back. Plus, my new one has a remote panel so I can monitor digitally the batter reserve.
There is a reason this is a spendy install if you choose not to do it yourself. If you do, you will understand what I mean