Why?
Your tent should still be water proof. Unless sun has damaged it.
I would hose it down with a garden hose. If the water runs off still repels water.
Dirty spots maybe I would use dawn dish soap.
I would call the manufacture and ask them before I would use Thompson’s on it.
Seems like I read that Thompson’s flakes off. Never used it.
Proceed with caution.
Can You waterproof canvas wall tent?
If you feel that you need to retreat your canvas wall tent for waterproofing, there are right ways and wrong ways to approach it. You should purchase a product made specifically for this purpose, such as Canvak. People have used other products in the past, such as Thompsons Water Seal or other types of wood sealants.
https://www.davistent.com/product/waterproofing/