Does the sub-panel seem over loaded? If so, I could keep the water-heater in the main panel and free up space in the panel another way.
Seems reasonable to me. Most of the equipment won't draw anywhere near the overcurrent rating, at least not during normal operation. Motor loads will draw a higher current on start, but you shouldn't have a problem.
I know I need four-strand wire to run to the sub-panel (2 hot, neutral, ground) but copper or aluminum and what gauge?
You can use either copper or aluminum, though I recommend copper for DIYers. Copper is quite a bit more expensive, but it's easier to work with (in my opinion). If you feel confident working with aluminum conductors, you can save some money using it.
I've covered the topic of feeder sizing here, so I won't go into detail. If you're using copper, you'll want to use 3 AWG conductors. If you choose to use aluminum, you'll need 1 AWG conductors.
If you want to run a single cable, instead of individual conductors in conduit. You can buy what's called 3-3-3-5 SER cable (1-1-1-3 for aluminum), which will contain three 3 AWG conductors (hot,hot,neutral) and a 5 AWG grounding conductor.
When I run the wire along the floor joist, does it need to be secured to the joist or can it just hang there and rest on the drop ceiling? Seems like it should be secured to the joist with wire hanger or something.
You'll have to attach the cable to the joists, using 1 - 1 1/4" staples or other approved means. Check the packaging, to make sure they are rated for the size cable you're using.
What are the things about this project that I don't know that I don't know. :) These are the scary things IMO...i.e. the questions I don't know enough to ask.
The cable you'll be working with is thick and heavy, and it's not going to be fun pulling it. You'll probably want a couple helpers, to help you wrangle it.
Make sure all your connections are tightened to the manufacturer's specified torque.
If you choose aluminum conductors, make doubly sure you tighten the connections. And don't forget the anti-oxidant.
Come back a day or two after the panel has been put into service, and tighten any connections that need it.
Don't forget to remove the bonding jumper between the grounded and grounding bus bars.
You'll need clamps big enough for the cable, to secure it to the panels.
should I put a 100 amp breaker in the sub-panel to act as the "main" for the sub-panel? Or is the 100 amp breaker in the main panel sufficient?
You can usually pick up a main breaker panel, for about the same price as a main lug only (MLO) panel. In my opinion, unless the secondary panel is next to; or within sight of, the main panel. You're better served to install a main breaker panel. It simply offers better protection during maintenance, or other work within the panel.
For example. If you turn off the feeder breaker in the main panel, and start working in the secondary panel. Somebody could easily come along, and flip on the feeder breaker. Since you can't keep an eye on the breaker, you can never be sure the panel will be dead. (unless of course you're using a lockout like you should).
If the secondary panel is in a separate building or structure, then you either need a main breaker, a main disconnect, or the ability to disconnect all ungrounded conductors within 6 or less hand moves.
Best Answer
Yikes. All your load is 120V.
The 30A RV is almost certainly a "TT30" which is 120V/30A.
Receps are all 120V obviously. The good news is, receps are 0 amps. The bad news is, things which plug into receps are various amps, and since you haven't discussed what those will be, we have to punt over to the usual assumption of 180 VA per recep, or 1800 VA the bunch, or 15A.
120V electric wall heater will be 12 amps typically. However presumably you will not be running it at the same time you are maxing out the RV. You won't be running the A/C at the same time as the heater, and it's less than 12A, so we'll count it as part of the heater's allocation.
There are too few simultaneous loads here to be able to effectively balance them on the two 120V phases included as part of 240V. Therefore while we can try to balance them on the 240V, we can't count on that balance existing. Therefore we must think about voltage drop for a single load. The worst case (for balancing and voltage drop) is the RV is maxed out @ 30A and nothing else is on. That must be the basis of our voltage drop calcs.
(so for folks who do voltage drop calcs... just plugging 240V and breaker trip into your friendly neighborhood voltage-drop calc won't tell the whole story. Loads this lopsided require thinking about drop in instances of pure 120V load.)
It's a pity; if only we had a way to guarantee balancing of the 120V loads, we could do our calcs based on your 240V draw, and the wires would be a lot thinner. A 10 KVA or even 5 KVA transformer could do that... but realistically, unless you get lucky on Craigslist (and know exactly what you are buying), the transformer would be more expensive than fatter wire.
Failing to run this 240V would be nuts
So you might think "Why not run it as 120V-only?" Because your future mini-split won't like that. And you may get a bigger RV someday.
I think we need to plan for 40A @ 240V of service... but also watch our voltage drop at 30A @ 120V. (24A [80%] for the RV and 6A for other misc loads.) 3% is a wire salesman's ideal but try earnestly to keep it under 5%.
So this looks like #2 aluminum.
#3 aluminum would work, but it's generally a unicorn.
And the limiting factor is the ability to support a lopsided 120V load based on the RV being most of it.
On the upside, since you're not in Canada, you could breaker the #2 aluminum at 90A. That means it will be totally ready for that mini-split and larger 240V/50A RV should you ever get one -- at 60A actual, voltage drop @ 240V will only be 3.3%.
If you wanted to super-chintz this thing, you might be able to swing #4 aluminum, but under certain conditions voltage drop would be noticeable. The cost differential #2 vs #4 will be tiny compared to total project cost.
Also, get a BIG panel
This wire can be breakered at 90A, which is enough to run a pretty big house. Given how ridiculously cheap breaker spaces are, and how expensive and project-blocking running out of spaces is, a 30-space panel is not excessive. Disregard number of "circuits" in a panel spec; that number is useless. So a 16 space/32 circuit panel is only a 16.