Since you would be dealing with maintaining a gas tank I would have all electric except a gas fireplace. 

If you don't care about income and are more of a capital gain kind of guy, prices are already high (as you noticed) and there's plenty of land available to buy so I'm not sure how much is left in the tank.

My daughter has been fishing in the creek for two days and never caught any fish.

Willing to exhaust all options as this is a big fish. 

My wife and I have overhauled our personal budget to reflect the 'pay yourself first' approach (allocate in order: income comes in -> invest -> donate -> fixed expenses -> variable expenses -> fun $$).We are looking to see what experienced people generally recommend for a holding tank for that 'invest' allocation while we target our first real estate deal.What financial vehicles have you guys used that works well for this?

Any grandfathering will be lost when one of those tanks fails. 

I would also consider being a bigger fish in a smaller pond.

In addition to that, the septic tank is undersized for the size of the home.

Quote from @Johnny Lynum: Quote from @Billy Smith: I like SFH in the A-B area rent it you do the  upkeep  doing as much as possible your self to help max your rent out .I like to pay down the note as fast as possible .If in a C-D area I would flip the SFH wash your hands of it.SFH are generally easier to unload if you get in a cash crunch, I like staying in the first time home buyers level of a SFH  keeping it in a hot zone .If I were a BIG fish I would do deals at various level's of course after  doing the home work . 

Look at just today, everyone thought stock market this year was going to rocket then china comes out with AI allegedly superior to ours and tanks tech stocks.