whats the biggest and heaviest and slowest mobile artillery in the world, and is heaps long range//?

Answer 1

Well, you are going to have to define heaps to get a better answer - the web says it is a short range weapon.
The answer to the rest depends on how far in the past you want to go. Germany and other countries developed huge guns that were used in WWI and WWII that had to be moved on special railroad lines and could only be fired from special pads that took tens of men a couple of weeks to create. Biggest might mean the seige mortars Germany took into France and Holland (big diameter, short barrel) or the Long Tom and Paris Guns with very long barrels but not very large diameter to go long distances. The USA had a long gun that required special tractors that connected at both ends to lift it and then it was driven from both ends. A separate heavy mounting disk had to be carried on other trucks and assembled before it was lowered. Today big artillery is obsolete because missiles can take it out while it is still being assembled.