No, your truck's support frame rotting out and causing your fuel tank to fall out is not normally covered by other than collision (OTC) or comprehensive auto insurance coverages.

Comprehensive covers your vehicle for losses resulting from incidents other than collision (OTC). For example, comprehensive insurance covers damage to your car if it is stolen; or damaged by flood, fire, or animals. This optional auto insurance coverage pays to fix your vehicle less the deductible you choose.

There are exclusions to OTC, mainly normal wear and tear and mechanical failures that are not due to a covered event or loss. If the support frame and fuel tank were not damaged due to an accident or other incident covered by your auto insurance but due to normal wear and tear then the repairs needed would not be covered by your auto insurance policy.

If the frame rotted and deteriorated due to water damage from a flood then you may see if your OTC coverages will cover your truck's repairs by making a claim with your insurance carrier, but it would be doubtful that the truck would be covered since the frame had time to rot and thus would appear that the water damage would have been from some time ago.

You can read through the terms of your OTC auto insurance policy and speak with your agent to find out for certain if any of the repairs that need to be made to your vehicle will be covered since state laws and insurance policies do differ.

Likely to cover the repairs that you need to the support frame, fuel tank and other various attachments, you would look at the truck's warranty (if it is still covered and this damage falls into their coverages). Or if you have mechanical breakdown insurance (MBI) on it you could see if these parts would be covered, though again normal wear and tear is not usually covered by this type of insurance either.