Well first, welcome to the forum! Very sorry to hear about your grandfather, best of luck to him in his fight! I take it from your profile the van is a 1996? What engine does it have? Basic rundown on the codes:
P0141 - there is heated circuit in the O2 sensor that when it gets to operating temperature, the sensor then switches depending on the amount of exhaust around it and if the computer determines it took too long for the sensor to respond, it will set the code. In most cases, if the problem can't be solved by checking the wiring harness/connectors, it's best to replace the sensors because there is no repairing them if it is an internal sensor problem.
P0138 is your downstream sensor, behind the catalytic converter and there again, unless the short can be repaired, replacing the sensor is the way to go.
P0720 - are you having any transmission problems with it? That sensor should be easily accessible to replace. We have a member on here, ramvan2500, who is excellent with these kinds of questions and if I know which motor you have, I can move your thread to that area, but, for now, I moved it to the Dodge Ram Vans
section where I'm sure ramvan2500 will see it next time he's on.
Again, welcome to the forum, you should jump on over to the Newbie Checkin
and introduce yourself!