Is it normal to pay a house/pet sitter?

Yup, really :) things like chemical toilet dump points, water refilling stations, camp sites etc. Australia has A LOT of that stuff all over the country. The only places you struggle are the really rural bits, but this is precisely why you need the camper. You can load up on everything you need and then just go off-grid for the super rural parts. You need somewhere to sleep at the side of the road sometimes too, because there will often be more than a whole day drive to the next point of civilisation, so having a camper is very useful for that. You could do it with a tent but that massively limits your driving as you need daylight to erect the tent, and also it's sweaty as fuck in a lot of Australia, and you may end up with inivited guests in your tent - such a deadly snakes and spiders, termites, ants etc.

I promise you that the campervan/rv is the way to go. Download an app called "wikicamps" when you get here. It has all of the information you need. It shows official and unofficial camping spots, where you can refill your water, where the fuel stations are, where there are toilets, where there are points of interest/monuments etc. It basically plans your entire trip for you. People leave comments too, so say, for example, you're driving down a really lonely outback road and there's nothing around for miles, you can look on wikicamps and see all the good spots to pull over to sleep, and you can click into them and see comments like "creepy as fuck, a weird guy hangs around here, do not camp here" or "lovely camping spot. Highly recommend, just make sure you park around the back of the hill" or whatever. It's a great app.

By round the outside I literally just mean drive all the way around the outside of Australia. Follow the coast. Take your time and weave inland for anything in particular you want to go and see. It doesn't matter which coast you start on, although east coast is probably good if you're flying into Sydney.