Motor home maker Winnebago Industries Inc. said today that it would buy privately held Grand Design Recreational Vehicle Co., a maker of towable recreational vehicles, in its biggest-ever deal, sending its shares soaring. Reuters reported that the cash-and-stock deal, valued at $500 million, will give Forest City-based Winnebago a bigger foothold in the growing towables market and create a company with revenue of about $1.4 billion. About 37 percent of Winnebago’s revenue will come from towables as a result of the deal, compared with about 10 percent now, Chief Executive Michael Happe said. Winnebago’s stock price was up 23 percent by midafternoon, at $29.