Wayne: Welcome to IRV-2
The algae/slime is a by-product from bacteria and fungus growing between the water and the fuel. Some of the bacteria may still be in the fuel and coating the tank..
Check with that diesel shop and see if they added a biocide to kill the bacteria. If not, you probably need a biocide to keep the algae from coming back. There are several brands, Power Service makes "Bio Kleen" that is supposed to be effective. (It is an "insecticide" of sorts so don't overtreat) It's also a good idea to stock up on fuel filters until all the algae producers are removed. Getting rid of all the water was a good first step. BIO KLEENŽ DIESEL FUEL BIOCIDE: A dual-phase biocide to kill microbes, bacteria and fungus.
Usually at the first sign of losing power, replacing the filter will get you back on the road.
Once the tanks are clean, you probably won't need to continue the biocide, try to always store the tank full of fuel, less room for condensation to form.
Hope this helps