You can always build a "Cheater cable",yourself,but I recommend one of the following solutions, because you really do want to get the full 30Amp capability for your coach, and at these currents, you want to get cables that are made right (no offense) using the right wire size, and connectors..
1- I recommend that you go to your local RV parts store (like CW). and get a CHEATER BOX. My coach has 50-Amp input, but I also have the following; a-50-30Amp cheater cord (of course, for those CG's with just 30-Amp service), and a 30-20Amp cheater cable(for when I visit a friend or relatives house for a night).
But, in addition to these, I have a cheater box that has 50-Amo output connector, and for input, it has a 30-Amp as well as a 20-Amp input. With it, I can hook to two sites, when allowed, and get my full, 50-Amps for my RV.
The great thing about this type of solution, is I have all standard cables and a box that takes care of utilizing proper phase connections into my RV.
A box like this, and a couple of pigtails with the right connectors, and you will have a SAFE and PROPER power supply from your generator's 2-20-Amp outputs, into your RV. system.