I'm not sure if you are talking about a steel or soft bagger, but i have the grass gobbler (steel). I buy 45 gallon bags from lowes. They are a perfect fit to cover the back of the gobbler and dump. Tie the bag and throw it on the truck.
I dump into a large garbage can. 3 bags per can. When full I grab the can and kick the base with my foot to throw it to my shoulder. With leaves two cans,one in each hand to carry to the truck. It is easy on my back this way, not going over the side of the truck with the catcher. Not having to bend over to lift the barrel. Home depot has strong 32 gal. barrels for around $20.00
I have both a steel Gobbler, and a cloth bagger for my 36" w/b. For me, the best method to get clippings from mower to trailer is a tarp, two large dumps per tarp. I use 7X7, white plastic tarps from Landscape Supply (CA). They are just the right size for my height -- pick up all four corners and swing the tarp over my shoulder. The weight of the tarp is almost negligible (unlike a barrel), so dosn't add much weight to the load, and is very easy to carry back to the mower.
For my use, the 7X7 is the ideal size. A bigger one means the load is too far down the back of my legs, any smaller means that the corners don't come together very well. I'm not a strong person, so perhaps I am unable to carry as much as some of the rest of you.
first I don't mess with garbage bags. Waste of time and money. I lay a tarp in the bottom of the pickup bed before dumping any clippings into it. I unload the truck by pulling the front of the tarp over the top and rolling the load onto the ground. I will layer several tarps so I can handle the load by myself.
I also use tarps to dump on if I don't want to go back to the truck to dump. I use smaller tarps for this so no one tarp gets too heavy to throw into the truck. I tie the corners together in pairs before dragging it back to the truck. This keeps the load on the tarp and makes it easier to handle when throwing the thing into the truck.
I also use these need bags I get from Menard's (like home depot). they are made out of the same stuff as the plastic tarps, and are about 30 gallon size. they are collapsible, have a large oping and stand up by themselves once you get something in them. And the best part is that they only cost a couple of dollars each after a rebate. and they usually last at least a whole season. I think they are rubbermaid brand.
As far as getting the grass to the truck, we usually just put the mower in high gear and run it to the truck to dump it. In certain instances we have put it in landscape bags and brought the truck over when we were finished, but that was only in the fast growth season where there was actually a lot of grass where the 36 was mowing. I dont know of too many large areas mowed and bagged with a 36 in mower. Sounds a little impractical to me
To answer the question posed above ... I tie the tarps, corner to corner, and carry to my trailer. I use a pickup with a cap (store all my hand tools, trimmer, blower, ect), with a 6 1/2 X 12 trailer. I use the front four feet of the trailer for debris, the rear eight feet for my equipment. I work solo, only mow 900-1200 per year, so can't afford a dump truck, or second vehicle to pull around dump trailer.
Handling the clipping is a real pain, but some customers demand that part of the service (please -- no starting a long list of posts about "why are you bagging at all - just mulch"). I have several HD barrels on the trailer, put clippings in there directly from the baggers, or leave the tied tarps in tact, getting them home. I have a debris trailer at home to dump the barrels and tarps. Once per week, or as needed (trailer hauls 3 cu yds), I haul the clippings to a dump, and fork off.
I know, I know -- much heavy work, and TIME. But, when one barely makes a living at this work, spending large money for alternatives is just not possible. I am unable to implement all the suggestions made in these kind of threads, "... buy this," "buy that," etc. We who are small operators don't have the financial resources to spend lots of money, just "because it would make things easier." And, I am 62, so cannot plan for a long term (5-10 year) lawn service business. Buying up much equipment, with strong possibility of a year or two of use, then selling isn't a good idea -- can't recover all the cash.