Draw bars are replaced on vans all the time just speak to a repairer, it's no big deal other than the COST. It's just normal RHS there's no special welding required or stress reliving needed. You can also use duragal tubing which a lot of van manufacturers use today in place of cold dip gal.
I'm with
@achjimmy - it's no big deal replacing the drawbar just not worth the cost as I found going down the path of a drawbar reinforcement for ATM upgrade.
The draw bar is normal RHS - duragal C450 is probably stronger than what's used on the expandas... engineer based prelim calcs on C350 or less, unless Jayco could/would confirm what grade of steel is used - I didn't bother going there...
Any way if doing it 'legal' for rego and insurance (in QLD) I reckon put aside $1000-$2000 for your draw bar replacement.
$200 for RPEQ engineer to run the calcs and $200 for certification of upgrade and issue of TMR mod plate. I gave up at this stage as I was told the Jayco drawbar was at the limits and the engineer wouldn't sign off on any further loading without additional strengthening - I.e. not due to welds but due to the RHS sections not having the capacity / deforming. Hence a further $500 - $1500 estimated to have a boily reinforce / replace the draw bar to carry the the extra ball weight was not worth it to me.
That's a couple of grand towards the next van, but the extra 300kg of cargo capacity whilst being fully compliant was originally enticing until the price point.
I'm sure you could engineer a draw bar longer than 300mn if so desired - it would need to comply with VSB1 force constraints and ADRs. It's your money, it's definitely doable.
Cheapest option would be forget about rego and insurance and go agricultural... Get some durgal and get a mate to replace the draw bar... Might affect your ESC and culpability if you cause an accident though.
Good luck with fitting the bikes in. Transmod in Maryborough were a great help and they do engineering designs for other vehicles and truck trailers too. I didn't end up using them given the they answered all my questions before hand and thus I canned the project.