You could remove the tires and take to your local Discount Tire, Americas Tire, Costco, Walmart or other supplier but you may want to call and see if they have trailer tires in your size as they may need to order them.
The tires now are 6 years old at best and should be replaced regardless of milage. Tire companies warn of keeping tires over 5 years or so because of possible internal breakdown of the structural members.
Black, Gray and Fresh tanks should be good to go but you will want to sterilize the Fresh with a chlorine bleach mix and run it thru the system first though.
Pressure washing shouldn't be a problem but don't get closer than 8-10" especially around windows.
You might want to wash by hand afterwards.