At end of lease term, they left $20k in damages and skipped town. 

Proper Insurance is the best out there for STR and covering party damages, but I'm not sure what they would think of a mobile home.

I never checked that room, so the damage was there probably before their lease started.

I walk through make sure that there is no obvious signs of damage, anything beyond wear and tear. 

Our concern is that fixing the windows could open a can of worms if there was possible water damage to the structure due to the poor condition of the windows.

Worst case scenario, they drain the water and find cracks or more damages and costs rise.

I will share my "Most Expensive Lesson" in the comments.To kick things off, here are ten examples of expensive lessons or mistakes in real estate investing:Underestimated Repairs: The classic pitfall where the cost of repairs and renovations far exceeds initial estimates, impacting the overall budget and profitability.Tax Liens: Failing to account for or being unaware of existing tax liens on a property can result in unexpected financial burdens.Contractor Liens: Not settling payments or disputes with contractors can lead to liens against your property, complicating sales or refinancing.HOA Fines: Overlooking or violating Homeowners Association (HOA) rules can lead to significant fines and headaches.Bad Loan Products: Opting for loan products without fully understanding their terms can lead to unfavorable financial conditions, such as higher interest rates or unfavorable repayment terms.Ignoring Zoning Laws: Investing in a property without a clear understanding of local zoning laws may restrict its use, affecting your investment strategy.Overpaying for a Property: Lack of research or getting caught in a bidding war can result in paying much more than the property's worth.Neglecting Due Diligence: Skipping thorough inspections and background checks can uncover unpleasant surprises after the purchase is finalized.Poor Tenant Screening: Failing to properly screen tenants can lead to unpaid rent, property damage, and costly evictions.Underestimating Market Risk: Not considering market fluctuations can lead to investments that don't pay off as expected, especially in volatile or declining markets.We've all been there in one way or another, facing setbacks that seemed daunting at the time.

I'm worried about him damaging my property.Any advice would be welcome.

In metros perhaps there is always enough water damage, mold outs, fire damage etc to leave a limited supply of these props.

For small areas, a bleach solution can work, but be cautious of potential surface damage.