Legal or safety issues carrying diesel fuel containers in RV

As long as the basement hatch is separate enough from the cabin to keep the smells out I don't see a problem. If it's connected somehow I'd worry about the stank. There are a ton of options for hanging it on the outside of your motorhome if the smell issue is a problem, off the bumper, spare tire, etc.

rogerdddddd, re-read and think about what you just asked after reading this:

You buy diesel and gas at a fuel station, dispense into a fuel can then put it in the back of your car or truck and drive home, or whereever you need to with it. You fill your cans for the RV and drive where ever you need to with it.
You and everyone else in the world, literally, has been doing this since you could drive and needed a can of gas for the lawn-mower. (If your one of the few people who's life experiences hasn't allowed you to do this, because there's always one, realize that everyone else does.)

Short and long answer is no problem.

I have seen fuel (gasoline or diesel) carried in 2-3 different places outside a class "C":

1: With a Jeep Wrangler toad soft-top, the fuel canisters were both places in the toad, and a few were bolted to the back, allowing for 20-30 additional gallons.

2: A rack was placed on the roof, with the cans attached to that.

3: A new bumper was welded on (the factory bumpers will shear off very easily), and the fuel cans mounted on that.

4: A rear hitch mounted cargo rack was used.

Of all of those, if one hasn't done it already, I'd go with the hitch mounted cargo rack.

DOT says you don't need a placard for diesel if it is non bulk. That is defined as "Non-bulk packaging means a packaging which has:
(1) A maximum capacity of 450 L (119 gallons) or less"

I would make sure to carry them in a well vented compartment. While not as bad as gas, the diesel fumes could build up and be explosive.

There was a news piece recently on people in the Vancouver area crossing the border to get gas, and putting gas cans in their trunks. One got rear ended and blew up.

https://globalnews.ca/news/5241062/peace-arch-border-fiery-crash-gas-cans-safety/