Discount tire says 3 years for ST tires. I personally never had any ST tires make it 2 years.
Michelin says their LT tires can go up to 10 years. I replace mine at 5 years..But I am in Phoenix AZ where the sun kills rubber.
How good is your full coverage insurance? A blowout can cause up to 5000.00 in damage. And when it happens it WILL ruin your trip. It may even cause an accident. After 5 years, no tire manufacturer will accept any responsibility for damage.
You have spent a lot of money getting into RVing.
Tires ARE a maintainence item. Don't cheap out on them. If you do, it may end up costing far more than a set of tires.
On another note: I often am involved in the ST vs LT tire debates. My position is that the reason we have ST tires is that there is a market deamnd for cheap tires.. Most are loathe to spend as much money on TT tires as they will for their daily driver. This opinion is poo, pooed by the ST tire proponents.... Many of the opinions in this thread bear out my position.