You can get out of a rental situation with Reliance and you can even do it without paying them anything, but they won't make it easy for you.
They will try to charge you a removal fee of about $100-150(iirc), to come get their own units. They will also try to charge you an account closure fee of $40-50.
You get nothing for this money if you give it to them. They will threaten you with all kinds of toothless nonsense if you don't pay these charges.
Have the old tanks disconnected by a tradesperson and your new ones installed, then tell Reliance to come pick up their stuff. They will tell you it's your problem and to pay them to remove the tanks. Just refuse to pay them and tell them you will store the tank for free for 30 days(or whatever time works for you). After that you will charge them $50 a month for storage. They will complain and threaten you, but they will send someone by the end of the 30 days to get their own tank.
As far as replacement tanks go, do you need six separate tanks for 6 separate buildings, or are there more than one in each building? If you can consolidate them, I'd look at buying 2-3 high efficiency GSW condensing gas water heaters to replace the 6 standard units, if you can consolidate them. The High effic. Ones cost more up front but save a lot in gas costs. They're 90% efficient compared to 70-75 for regular ones. They also have faster recovery times. A 50 gal one will heat to 140 degrees from cold startup in 10 minutes.
A 50 gal one will easily supply hot water to 2-4 units depending on unit makeup; 2- 2-3bedroom, or 3-4 singles.