I want to add a hot water recirc line to my trailer

Don't listen to the naysayers. I did the very mod you are suggesting and it works perfectly fine. Instead of cutting a new hole in the tank I cut the pump supply hose right close to the tank and installed a tee. link
It takes about 10 seconds of operation to get hot water up to the bathroom faucet. You won't get hot or even warm water into the fresh water tank unless you continue to let the bypass run.