Sounds like you're very knowledgable.

What do you know about the 10L tankless propane water heaters? They can be bought for $200 - $300 bucks. I'm building a one room cabin with one shower and a sink. Will those provide enough hot water for a small cabin? Just weekend use, was going to put two 30 or 40 pound tanks to supply gas to it.

If I were going to do a whole or even part house Generator, I would do 500 gal. You can bet with about anything you put in, 2 gal per hour average. Thats my guess runing it with what you need, not every light in the house.

In round numbers my guess you are going to run through 50 gal. every 24 hrs. 250 is not going to last all that many days. Some areas on the coast can go long periods of time before power gets restored.

I've been lucky, we lost power by during Ike for a couple of days when I lived in Pearland. Bro inlaw that lived couple of miles away went three weeks without power. Some people in the area went 8 weeks.

During Harvey and 56 inches of ran in our backyard our lights never blinked, but others were without for several weeks.

Where ever we land for retirement I plan to have a whole house or near enough whole house propane generator. Along with large stand alone propane take just for the Gen.

That's it !

In the last 16 years I've had 4 major events lasting up to 3 weeks of outage. I do want to stay home and continue to live in somewhat moderate comfort. Lake Charles, Louisiana has had 3 events within a year . . . all major devastating storms destroying central power infrastructure.

Living on the Gulf coastal plain takes a little gumption and forethought. Pat can see the water where he lives. He must take into account storm surge plus winds. I'm sure you can understand wanting to stay at your home and begin repairs while maintaining a somewhat liveble standard is more than "feeling cozy".

I really consider having an alternate energy source as needed, a home improvement, and raises one's property home value if ever that is a factor. But I've had to be off grid several times too.

Anything that will give you over 3 gallons per minute you will be fine. A good estimate for each water outlet is 1.5 gallons per minute, This will give you ample water for that small of enough cabin space.
I have been in the propane business for 28 years come next month.

Here is yet another variable to consider. 1- if you are in a flood zone you will have water come into the dome of an underground tank and cover the regulator to the tank(s). They will need to be replaced as water has entered into and onto the diaphragm and will cause major problems with gas pressure going to the home. Even with an above ground tank if the water gets higher than the tank itself you will have the same problem with the regulators.
2- If you go with either the underground or above ground tank system in a flood zone you will need to also get an anchoring system for those tanks or they will float away.
The other issue will be your second stage regulator or low pressure regulator at your generator, that will also needs replaced if water intrudes the regulator.
One main issue with using propane is floods, just about everything else is pretty plain and simple, flood zone tanks are another animal. National Code and your local codes will determine just how much they will require to complete installations in a flood zone. Some local zones prohibit them.

Those work great for campers, they hold around 24 gallons each , so your gas capacity will be 48 gallons. A comparison is the normal #30 lb cylinders attached to campers, they will hold around 6 gallons each, thus giving you a 12 gallon supply.

Generator prices are shooting up I hear. With this heat now...power grids are working hard. Tx power companies are now urging folks to turn off appliances not needed.
Looks like this tropical disturbance further south just may come G-town way . Stay alert.

Ok
How tricky is the switch to install?
Is there a manual cut out that would work just as well?

on the bigger units like generac whole house the switch is the brain of the unit, tells it when to come on etc. and I doubt you can do a manual start on them, need to call generac or an installer on that one.
the auto switch is really the way to go imo, don't have to screw with anything when the power goes out.

1000 gallon tank is 10’ as long as there is not another tank within 25’. Note 2 NFPA58

Actually you can have 1000 gallon tanks beside or end to end as long as they are 3' apart.
The 25' rule is property lines and from the house or an electrical unit. I think the difference you are alluding to is for underground tanks only. Then you may have them at the 10' distance.
But as to a single container, yes #3 rule can be applied as long as you adhere to the 25' rule with no other container in that space.
Now...to cover yourself you will have to be at least 15' or more from your property line with a 1000 gallon tank. Why? You next door neighbor might set a 500 or smaller tank 10' from the property line. This means you will need that 15+ feet to now cover the 25' for the code. If not then you will have to move your tank back 25' from your house and still stay 15 or more feet from your property line. In other words....it just becomes a mess changing out you gas lines and finding another spot.

Where would you stick the tank at?
Big rains gets pretty swampy. Good neighbor is below, douche is above.

1-What size tank?
2-One or two tanks?
3-You have to remember that to get it refilled, you have to be around the 110' mark from your driveway for the hose to reach the tank to fill it. The hose is normally 125, long but you have to take in considerations of corners and other obstructions. You do not want that truck that weighs over 33,000 lbs to go across your lawn, it will leave deep tracks and break any lines under the ground not deep enough to support that weight.
4- Is this in a flood zone?

We have a 500 gallon propane tank to support our 22 KW Generac 1800 sq ft house. They only fill the tank to 80% to allow for gas expansion in the summer heat. We knew after Hurricane Rita that we could not live on the edge of a national forest where the electrical lines go right through it, that we HAD to have a standby generator. When our power goes off, in about thirty seconds the Generac is on and we are back in business.would not live a mile from a paved road without it.