I like your thinking. Now, no offense intended, but I think your expected gain is way off. I really recommend doing a lot more research and math. I'm basing this on you mentioning you could get 80 nights of rentals at $100/night each year, and that would pay off your initial investment of $30,000 in 4 years -- I.e., that you will earn $8000/year for four years = $32,000. This is incorrect on many levels:
1. Even if you get your optimistic rate of $100/night, you will not get to keep anywhere close to that. You might be lucky to keep $20/night of that after expenses are considered. RE experts suggest using a "50% rule" for long term rentals -- short term rentals will have much higher costs due to daily cleaning, key turnover, management, excessive wear and tear, advertising, insurance, transient rental taxes, vacancy, etc. Note that this excludes any financing costs (that is, if you have to carry a mortgage, that is even more off your bottom line).
2. If a full house rents for $125/night, I think $100/night for a tiny house is severely over-optimistic. I'd suspect maybe $80/night as the maximum you could get, lower if there are other alternatives for couples or single travelers like hotels and B&Bs.
3. Septic and wells for the additional houses will add a lot more to your initial investment, maybe $10-20k +.
4. You must also consider the opportunity cost of your invested money. If you invested $80k (two tiny houses plus septic/well) in an index fund instead, you might realistically expect a 7% average annual gain. That's around $6k annually with no work at all required. If you can't make more than this in cash flow by doing these short term rentals with higher risk and significant work, it doesn't make sense to even consider.
5. Finally, what happens when you eventually want to sell? Will you find lots of interested buyers to buy this property with several tiny house rentals on it? Presumably no one will want it unless it's a wildly profitable business, but if it's wildly profitable you wouldn't likely sell anyway...
Anyway, good luck, just think carefully about the real numbers involved in the possible venture.